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Furnace Making Loud Noise When AC Is On – Causes & Fixes

Experiencing a furnace making loud noise when AC is on can be both unsettling and indicative of underlying issues. 

While it’s natural for some noises to arise as the system operates, especially due to ducts expanding and contracting, a consistent and exceptionally loud disturbance often points to more significant problems. 

These noises not only disrupt the peace of your home but can also hint at inefficiencies or potential breakdowns in your HVAC system.

When a furnace makes loud noises while the AC is on, it’s often a sign of underlying issues ranging from worn-out components, obstructions, or even refrigerant leaks. Addressing these problems promptly ensures efficient and quiet operation.

Furnace Making Loud Noise When AC is On – Short Guide

For many, the sound of a furnace humming softly in the background is synonymous with comfort. However, when that hum turns into a disruptive noise, especially when the AC is on, it’s a cause for concern. Such noises are more than just disturbances; they’re often indicators of underlying issues in your HVAC system.

The reasons can be many, from worn-out belts to malfunctioning blower motors to blocked air filters. Addressing them promptly not only ensures a serene ambiance but also maintains the health and efficiency of your furnace. After all, in the fashion world, it’s not just about looking good but feeling good in your space!

10 Reasons Why Furnace Making Loud Noise When AC is On

Every homeowner relies on their furnace to keep their space comfortable, especially during extreme weather conditions. However, when your furnace starts making loud noises while the AC is running, it can be both annoying and concerning.

It indicates that something might be wrong with the system. Understanding the potential causes of these noises can help you address the problem more effectively.

Here are ten reasons why your furnace might be making loud noises when the AC is on:

Worn Out or Loose Belts 

Belts are an integral part of your furnace, ensuring various components move synchronously. Over time, these belts can wear out or become loose, leading to unsettling loud noises. If not addressed, these worn or slack belts can lead to a cascade of problems, including reduced efficiency, overheating, or even complete system breakdowns. It’s like driving a car with a worn-out timing belt; sooner or later, it’s going to cause bigger issues.

Solution: Replacing Worn Out Components 

Over time, components in your furnace can wear out, leading to inefficiencies and increased noise levels. Recognizing and replacing these aging components is essential to keep your system running smoothly. Not only does it ensure the quiet operation of your furnace, but it also improves its overall efficiency, leading to energy savings.

Steps to Replace Worn Out Components:

  • Regularly inspect components for visible signs of wear or damage.
  • Consult the furnace’s manual or an HVAC professional for guidance on replacement parts.
  • Ensure replacements are compatible with your specific furnace model.
  • Consider upgrading to more durable or energy-efficient components if available.

Malfunctioning Blower Motor 

At the core of your furnace’s operation is the blower motor. It’s responsible for pushing air throughout your home, ensuring an even distribution of warmth or coolness. When the blower motor malfunctions, it can lead to a variety of loud and alarming noises, ranging from screeching to thumping.

The root causes can vary – it might be due to general wear and tear, electrical malfunctions, or other internal issues. Regardless, a malfunctioning blower motor disrupts the usually quiet and seamless operation of your HVAC system, warranting immediate attention.

Solution: Regular Maintenance Checks 

The foundation of any efficiently running HVAC system is regular maintenance checks. These assessments act as preventive measures, catching minor issues before they escalate into costly repairs or replacements. By scheduling periodic evaluations, homeowners can ensure the longevity of their system, reduce operational costs, and maintain optimal performance.

Steps to Ensure Effective Maintenance Checks:

  • Schedule bi-annual checks with a certified HVAC technician.
  • Keep a maintenance log to track any changes or repairs made.
  • Address any issues or recommendations provided by the technician promptly.
  • Regularly inspect for any visible signs of wear, damage, or leaks.

Blocked or Dirty Air Filters 

The quality of air in our homes is something we often take for granted. However, ensuring this requires clean air filters. These filters play a pivotal role in ensuring efficient furnace operation by trapping dirt, dust, and other pollutants.

Over time, these filters can become blocked or excessively dirty. When this happens, it causes your furnace to overwork, leading to potential strain, loud operational sounds, and reduced efficiency.

Furthermore, dirty filters can compromise the overall air quality in your home, exacerbating allergies and posing various health risks.

Solution: Cleaning or Replacing Air Filters 

Air filters play a pivotal role in ensuring the quality of air in your home and the efficiency of your furnace.

Over time, these filters can accumulate dust, reducing airflow and causing your furnace to overwork.

Regularly checking and replacing air filters can prevent this, ensuring a cleaner living environment and reduced energy consumption.

Steps to Clean or Replace Air Filters:

  • Check filters monthly for any visible dirt or blockage.
  • Clean lightly soiled filters using a soft brush or vacuum.
  • Replace heavily soiled or damaged filters with ones compatible with your furnace.
  • Consider using high-efficiency filters for better air quality and longer lifespan.

Damaged Ductwork 

Imagine the ducts as the circulatory system of your home – they are responsible for distributing warm or cool air throughout the space.

Over time, due to various factors like age, physical damage, or even pests, these ducts can become damaged. This damage can manifest as holes, cracks, or disconnections, creating loud noises as air moves through them. But the problem isn’t just limited to noise.

Damaged ductwork can significantly affect the efficiency of air distribution in your home, leading to uneven temperatures and increased energy bills.

Solution: Sealing and Repairing Ductwork

Ducts, being the channels for air distribution, need to be in prime condition. Damaged or leaky ducts can lead to inefficiencies, uneven temperatures, and increased operational noises. By sealing and repairing ductwork, homeowners can ensure efficient heating or cooling and reduce energy costs.

Steps to Seal and Repair Ductwork:

  • Regularly inspect ducts for any signs of damage or leaks.
  • Use mastic sealant or metal tape to seal any minor gaps or holes.
  • For significant damage, consider replacing sections of the ductwork.
  • Insulate ducts in unheated spaces to improve efficiency.

Obstructed Condenser Unit 

Outside your home lies the condenser unit, a crucial component of your AC system. It’s responsible for releasing heat from the refrigerant, ensuring efficient cooling.

However, its outdoor location means it’s exposed to various elements – debris, plants, animals, and more. When these elements obstruct the condenser unit, it can lead to loud operational noises.

Beyond the immediate noise issue, obstructions can hinder the performance of the condenser unit, affecting the overall cooling efficiency of your system. Regular checks and cleaning are essential to keep this component running smoothly.

Solution: Clearing Obstructions from the Condenser Unit 

The condenser unit, located outside the home, is crucial for the AC’s cooling process. Over time, it can become obstructed by debris, leaves, or even wildlife nests. Regularly checking and clearing obstructions ensures that the unit can release heat efficiently, reducing noise and ensuring optimal cooling.

Steps to Clear Obstructions from the Condenser Unit:

  • Periodically inspect the unit for any visible obstructions.
  • Gently brush away loose debris or leaves from the unit.
  • Trim any plants or shrubs growing too close to the unit.
  • Consider installing a protective cage or screen to prevent wildlife interference.

Faulty Expansion Valve 

The role of the expansion valve in your AC system cannot be emphasized enough. Tasked with regulating the flow of refrigerant, it ensures that the system operates at optimal pressures.

However, when this valve malfunctions, it can produce a range of unusual and concerning noises. These sounds aren’t just a mere annoyance; they act as warning signals of a deeper issue.

A faulty expansion valve doesn’t just impact the noise levels; it directly compromises the cooling efficiency of the system, leading to uneven temperatures and increased energy consumption.

Solution: Checking and Replacing the Expansion Valve

The expansion valve serves as a critical component in your AC system, regulating the flow of refrigerant and ensuring optimal cooling conditions.

A faulty valve can hinder this process, leading to inefficient cooling and louder operational noises. If you suspect an issue with your expansion valve, taking prompt action can prevent further complications and ensure the longevity of your system.

Steps to Check and Replace the Expansion Valve:

  • Listen for hissing or bubbling noises, common indicators of a faulty valve.
  • Consult your HVAC system’s manual to locate the expansion valve.
  • Turn off the system and consult a professional to inspect the valve.
  • If deemed faulty, replace the valve with a compatible model.
  • Regularly inspect the valve to ensure its optimal functioning.

Refrigerant Leaks 

Imagine the refrigerant as the bloodstream of your AC system, vital for its cooling function. When there’s a leak, it’s akin to a lifeline being cut off. This disruption often results in the furnace emitting loud noises, signalling a problem.

But the issue goes beyond just the auditory disturbance. Refrigerant leaks can dramatically reduce the efficiency of your system, making it work harder and consume more energy. Moreover, these leaks pose an environmental concern, as certain refrigerants, when released, can contribute to greenhouse gas emissions and harm the ozone layer.

Solution: Addressing Refrigerant Leaks 

Refrigerant leaks can be detrimental to both the environment and the efficiency of your HVAC system. Such leaks can lead to a decline in cooling performance, increased energy consumption, and potential environmental harm. It’s essential to identify and address these leaks without delay to maintain your system’s optimal operation.

Steps to Address Refrigerant Leaks:

  • Monitor the cooling performance; a sudden drop might indicate a leak.
  • Check for visible signs of leaks, like oil stains or frost accumulation.
  • Use a refrigerant leak detector for a thorough inspection.
  • If a leak is identified, consult a professional for repairs.
  • Once repaired, ensure the refrigerant is refilled to the recommended level.

Aging Furnace Components 

Every machine has a lifespan, and your furnace is no exception. Over the years, its components can degrade due to constant wear and tear. As these components age, they tend to produce louder operational sounds, which can be unsettling. These noises aren’t just signs of old age; they can indicate potential malfunctions or inefficiencies in the system.

It’s essential to conduct regular maintenance checks, ensuring that aging components are identified and addressed promptly. By doing so, you not only reduce noise but also extend the life and efficiency of your furnace.

Solution: Upgrading Older Furnace Systems 

As technology advances, newer furnace models become more efficient, quieter, and environmentally friendly. If you’re using an older furnace system, considering an upgrade can be a wise investment, ensuring quieter operation and improved energy efficiency.

Steps to Upgrade Older Furnace Systems:

  • Assess the age and performance of your current system.
  • Research newer models that fit your home’s size and heating needs.
  • Consult HVAC professionals for recommendations and installation.
  • Properly dispose of or recycle the old system.
  • Ensure regular maintenance of the new system for optimal performance.

Improperly Sized Ducts

The importance of appropriately sized ducts in an HVAC system is often underestimated. Think of the ducts as the highways for air in your home. When these “highways” are too narrow or wide, it leads to congestion or inefficient flow, resulting in loud operational noises.

Improperly sized ducts don’t just cause noise issues; they directly impact the heating or cooling efficiency of your home. Rooms might feel too hot or too cold, and the system may consume more energy than necessary. Consulting with HVAC professionals can ensure that your ducts are the right size, optimizing both noise levels and efficiency.

Solution: Ensuring Proper Duct Sizing 

The size of the ductwork in your HVAC system plays a pivotal role in its performance and noise levels. Improperly sized ducts can lead to inefficiencies, increased energy consumption, and louder operational noises. Ensuring the right duct size is crucial for the system’s optimal functioning.

Steps to Ensure Proper Duct Sizing:

  • Assess the current size and layout of your ductwork.
  • Compare it with the recommended size for your home and HVAC system.
  • Consult HVAC professionals for a thorough evaluation.
  • If resizing is necessary, seek professional installation to ensure accuracy.
  • Regularly inspect ducts for damages or obstructions that might affect performance.

Lack of Lubrication in Moving Parts 

Much like how vehicles need oil to run smoothly, the moving parts in furnaces require regular lubrication. Without it, these parts can grind against each other, producing loud, grating noises that can be alarming. This lack of lubrication isn’t just a noise issue; it can lead to faster wear and tear, reducing the lifespan of the components.

To ensure the seamless and quiet operation of your furnace, regular lubrication checks and applications are paramount. This simple preventive measure can enhance the longevity and efficiency of your furnace, ensuring it operates like a well-oiled machine.

Solution: Lubricating Moving Parts Regularly 

All machinery requires regular maintenance to function optimally, and your furnace is no exception. Over time, moving parts can experience wear and tear. Regular lubrication ensures these parts operate smoothly, preventing grinding noises and potential damage.

Steps to Lubricate Moving Parts:

  • Identify moving parts in your furnace, like motors or fans.
  • Turn off the furnace and ensure it’s cool before starting.
  • Use a high-quality lubricant recommended for HVAC systems.
  • Apply the lubricant to moving parts, ensuring not to over-apply.
  • Turn on the furnace and listen for any unusual noises indicating a need for further maintenance.


In conclusion, a furnace making a loud noise when the AC is on isn’t just an auditory nuisance—it’s a cry for attention. Whether it’s due to aging components, refrigerant leaks, or improperly sized ducts, identifying and addressing the root cause is paramount.

By understanding these potential issues and taking proactive measures like regular maintenance checks and timely replacements, homeowners can ensure the longevity and efficiency of their HVAC system, all while enjoying a peaceful and comfortable living environment.


Why does my furnace make a loud banging sound when the AC starts? 

A loud banging often indicates delayed ignition due to dirty burners or gas buildup, causing a small explosion when the furnace ignites.

Can a blocked condenser unit cause my furnace to be noisy when the AC is on? 

Yes, obstructions in the condenser unit, like debris or plants, can hinder performance, leading to loud noises during operation.

How do worn-out belts in the furnace contribute to increased noise levels when the AC runs? 

Worn-out or loose belts in the furnace can create unsettling loud noises due to misalignment or reduced tension during operation.

Is a malfunctioning expansion valve a reason for my furnace making noise with the AC on? 

Yes. A faulty expansion valve can produce unusual sounds by disrupting the flow of refrigerant, affecting cooling efficiency.