Sounds Your HVAC Makes That Shouldn't Be Ignored

Published 10/16/2023

Mr. Duct HVAC Sounds

There are several noises your HVAC system could be making, and if you're heard some of them, they can be a bit jarring. With a better understanding of what types of noises you're hearing, you can potentially diagnose the problem early.

Screeching or Squealing

This noise can be a sign of a bad fan belt in the blower. If this is indeed the case, that belt should be replaced. The sound also might mean that the motor needs lubricant—which you may or may not be able to do yourself depending on the furnace make and model.

It does take a special type of lubricant, though (not an ordinary household type). When in doubt, call on the experts at Mr. Duct.


One common HVAC noise is the rattle. This might simply be that your compressor is failing. It also could be a pulley with failing bearings, or a worn-out compressor clutch. Anyone of these types of concerns warrants the attention of a technician.

This is a sign that something is coming loose in the motor or blower. If the banging gets louder, it indicates that whatever was loose before has now completely broken off.


If the sound is coming from the ductwork though the heat hasn't been on for a while, this is a common noise. This is just the sound of cold metal ducts expanding from warm air rushing through. If the popping comes from a different source or doesn't go away, it could be time to call a technician to find out for sure what's really going on.


If you are hearing this common HVAC noise from the AC unit, it could be a malfunction in the compressor.

Other issues that can cause this sound include refrigerant leak that causes the unit to freeze or could just be a loose part vibrating. Either way, this is an issue that requires a professional to take a look and diagnose.


If you hear clicking coming specifically from the AC unit, it's usually the outside compressor or the inside air handler. Though there are various possible causes, a technician will be able to determine and diagnose the problem easily.

If you turn on the heat for the first time in months, a clicking sound is typically normal, as it indicates the system is working properly. However – if the sound is coming directly from the furnace, that could be an indicator of an ignition system malfunction. 

If the unit is clicking constantly or more than at start up and shut down, it may be that there are electrical control problems, and the clicking sound may even be caused by a defective relay or a thermostat issue.


The sound of booming happens when the furnace tries to light from a pilot light, but simply cannot light. Because of this, there's an increasing gas build-up within the combustion chamber that will suddenly ignite all at once.

This is not a minor issue, and will also require a technician to review the problem, which is usually caused due to the build-up of carbon particles. The chamber and the flue should be inspected and cleaned professionally.

High-Pitched Whistling

A high pitch whistling sound is essentially a warning noise. If it originates from the AC compressor, you should act swiftly to turn the electrical power to the unit off.

This could be an indication that the refrigerant pressure is abnormally high, or that your compressor is failing. In either case, as soon as possible, make a call to Mr. Duct to have a look at the unit. Do not turn on the unit until it has been repaired, because refrigerant leaks are highly dangerous to people and animals.


Hissing occurs, often times when the unit isn't even running – which means the refrigerant line is leaky, the compressor pressure it too high, or if there's an internal valve leak. It really depends on the exact location of the sound.

If coming from the AC compressor, an internal refrigerant leak could be the issue, and in this case, it is important to disconnect the power. If it is coming from the furnace, it could be a leaking air duct which will certainly require professional repair.

Help is On The Way

Mr. Duct services all makes and models, and because of this, offers experience with handling any type of unit. This means you don't have to have an inexperienced technician fixing or diagnosing your unit. Because we are a “parts on wheels” service, our repair vans contain common furnace parts, in an effort to minimize the need for multiple visits.

If you need to schedule a repair or diagnose a problem with your unit, call is at 800-501-4157 or fill out a form with your service request details.

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