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Furnace Making Knocking Noise? 5 Common Causes (+ Fixes)

Furnaces are designed to quietly warm your home, so loud noises coming from your heater are usually a bad sign. Hearing a constant knocking is the most common complaint homeowners have about their heating systems. If you’re experiencing this issue, this could be for several reasons.

A furnace will make a knocking noise when the weather gets colder or if it needs to be repaired. A continuous knocking sound is a sign of poor installation, a damaged fan, or worn down metal parts. Generally, repairs should be done by a professional, but there are some fixes you can do on your own.

In this article, I’ll discuss possible causes of the knocking sound from your furnace and how to identify them. I’ll also explain which problems can be solved at home and when it’s time to contact an HVAC professional.

The Blower Fan Has Become Loose and Is Shaking

The blower fan is the part of the furnace that sends the hot air throughout the ducts in your home. It is held onto the motor by a screw. If the screw becomes loose, the fan will shake, creating a knocking sound. 

How To Spot a Slack Blower Fan?

The fan is usually located at the bottom of the furnace. 

Repair Clinic suggests taking these steps to access your blower and check the screw safely:

  1. Turn off the heater.
  2. Remove the access panel(s) blocking the control board.
  3. Run a screwdriver across the wires to test if the electrical current has been turned off.
  4. Disconnect the wires joining the fan and the control board.
  5. Unscrew the surrounding bolts securing the blower fan, and take it out of the furnace.
  6. Examine the center screw to see if it’s secure.

Can You Repair a Damaged Blower Fan at Home?

A loose fan blower can safely be repaired at home. Although it’s always best to contact a professional, once you’ve gained access to the furnace’s fan, it only takes a few minutes to change the screw.

I recommend getting the make and model of your heating system first. This information will help you purchase the correct screw size to prevent the blower from becoming loose again in the near future.

Cold Weather Is Stressing the Furnace’s Machinery

Your furnace has to do double the work to warm your home during winter, especially if you haven’t turned it on in a while. 

Having to give more effort than usual can cause heaters to produce a knocking sound. This is nothing to be alarmed by and is expected during very low temperatures.

How To Know if the Weather Is Causing the Knocking?

If the tapping sound starts immediately after powering on your furnace but stops after a minute or two, it’s most likely caused by the weather.

There is nothing you can do about this. The knocking should stop when it’s warmer outside.

However, if the knocking continues or starts back up, that is the sign of a more serious problem.

The Air Filter Is Clogged and Making Noise

The air filter stops dust, hair, and other debris from traveling through your air ducts and into your home. Because these filters are constantly collecting dirt, they need to be replaced or cleaned every few months.

Neglecting to perform regular maintenance on the heater’s air vents leads to congestion. A clogged filter makes it difficult for air to pass through, which will result in some unusual noises

How To Know if Your Furnace’s Filter Needs To Be Replaced?

Disposable filters are designed to last for a couple of months. If it’s been at least 12 weeks since you last changed filters, they’re due for a replacement.

Things like fur balls, tin foil, paper, and small animals could also be blocking your single-use air filter. Get rid of the furnace filter immediately if you come across large items like these.

Unlike disposable filters, permanent air filters do not need to be replaced when blocked up. They can be cleaned and reused as long as they are in good condition.

How To Safely Clean Your Filter?

Mild dish soaps, spray-on cleansers, and filtered water are all safe options for removing grime from your reusable air vent. 

Soak the filter in your preferred solution, then rinse it with pure water until the water runs clear. 

Some people prefer to air-dry their filter before using it again, but it’s completely fine to put it back in place as soon as it’s done being cleaned. Hot air will dry the filter over time as it is being used.

A Damaged Fan Belt Is Hitting Other Parts

A fan belt that is cracked or broken will hit metal parts of the furnace as it rotates. Damaged fan belts should be replaced immediately to keep other components of the heating system free from harm.

How To Identify a Broken Fan Belt?

You can remove the furnace panel and check, but it’s possible to pinpoint a split fan belt without opening up your heater. 

The knocking from a damaged fan belt will be strategic. The scrapes and taps will sound like clockwork. Listen carefully to what sounds come from your heating system and how often. If you can predict the knocking, then your fan belt needs to be repaired.

How To Fix a Torn Fan Belt?

Servicing the fan belt in your blower is a delicate process. It requires the skills of a trained HVAC technician. 

Trying to repair a fan belt at home without the proper tools will severely damage your heater. Always contact a professional to solve this type of problem.

The Air Ducts Are Hitting the Walls

Expansion and contraction are two processes that take place in your air ducts when the temperature rises in your heating system. As the metal parts heat up, they begin to move around. So, a knock or a bang could be the result of your air ducts colliding with your walls or roofing. 

This type of behavior is normal and usually does not signal a problem. 

How To Figure Out if the Air Ducts Are the Source of the Knocking?

A knocking sound caused by your air ducts will only appear when the heater first starts or immediately after it stops blowing hot air. 

How To Stop the Air Ducts From Being Noisy?

An HVAC expert would have to help with this problem. 

Extremely noisy air ducts are a sign of a poor installation. As you’d suspect, solving this can be very dangerous; therefore, at-home fixes are not recommended.

How To Choose the Right HVAC Technician for the Job

Finding a professional to repair your heater is actually a simple process. When picking the perfect HVAC technician:

  • Do your research
  • Look at the technician’s certifications and licenses
  • Check their working experience
  • Read testimonials 
  • Ask for an estimate
  • Have a consultation
  • Request an on-site visit
  • Get everything in writing

Final Thoughts

A knocking noise coming from your furnace can signal a wide variety of things. 

Problems like a dirty air filter or loose blower fan can be solved at home once you have the right tools. Suppose you’re having trouble figuring out how to fix your heating system without any help. This Youtube tutorial by AMRE Supply provides a visual guide on minor repairs for your heater: 

For more problems like a broken fan belt or noisy air ducts, contact an HVAC technician. They are trained to tackle these problems as safely as possible.


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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