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Thump, thwack, clank. HVAC noises to watch out for.

If you’ve ever seen the engine light come on in your car or broken down on the side of the road, you know that feeling that comes over you. It’s the same feeling that comes over you when you hear a strange knock, bang, or rattle coming from your HVAC system.

Whether it’s your car or your furnace, a broken down piece of equipment will activate your mind into imagining a host of scenarios that run the gamut of financial woes, safety concerns, and just plain discomfort.

When you turn your heating system on, there are a lot of noises that you’re probably familiar with. You’ll hear clicking as the flames light up on an oil or gas system, the blower whoosh into action if you have hot air, and the pipes or ducts ping or knock as they heat up. You can almost feel the house get warmer by just hearing those sounds. We get so used to them that they become part of the ritual of turning on the heat. But sometimes you might hear a new noise. One that you’ve never heard before. One that sounds particularly terrifying.

In the following section, we’ll examine and discuss some of the possible noises that could indicate a problem with your HVAC system.

Thumping, rattling, banging, or clanking

These sounds usually come from the blower and could indicate that there is a problem with the blower assembly or the motor. Rattling noises are generally caused by something that has come loose. Louder banging noises when the blower is running could be a clue that something is broken or disconnected.

The sounds can also be an indication that the blower could be out of alignment and in some cases that the motor mounts have become loose.

Rattling can be caused by motor bearings.

If you hear these noises, shut the system down before the problem gets worse and have the motor inspected by a qualified technician.

Screeching or squealing

Unless you run a haunted house, screeching or squealing coming from the basement — particularly your heating system — is usually an indication of a bad belt or motor bearing. Belt replacements are generally not an expensive repair, but they should be replaced before they break.

In some cases, the problem can be solved with lubrication. Most blower motor units have ports for adding lubricants, while others are sealed. We recommend calling a technician, as household lubricants like WD-40 or 3-in-1 oil are too lightweight for this job.


Yes, “thwapping.”

“Thwapping” sounds like someone shuffling playing cards and is usually a sign that something is stuck on the blower blades or has come into contact with the blower motor and this is the sound it makes as the motor turns.

This issue should be addressed as soon as possible to avoid potential damage to the equipment.


You will generally hear clicking sounds when your HVAC unit is turned on or off. If the clicking is constant, it could be a sign of a defective relay or any number of electrical issues. Electrical controls may also be causing the relay to fail. Again, have a qualified technician look into the reason for this noise.

The noises and descriptions listed above are by no means scientific or complete, but the descriptions should fall in the neighborhood of the abnormal noises that can come from your HVAC system. Your home and your home’s HVAC system have their own set of sounds that you are uniquely qualified to identify. This also makes you uniquely qualified to know when something is out of the ordinary.

As usual, when in doubt, call in a pro to check things out before an odd or annoying noise turns into something more critical and permanent.


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