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What is that Noise? Common HVAC Noises Explained

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What is that Noise?

Common HVAC Noises Explained

All appliances make noise, like the dishwasher as it runs through a cycle or the whirring of the refrigerator motor. We understand those noises are normal and learn to ignore them. It’s only when an unfamiliar noise fills the air do our ears perk up. What’s that noise and is it normal?

An HVAC system controls the indoor climate by regulating temperature. It’s comprised of a heating unit (such as a furnace), a cooling unit (AC unit), a thermostat and ductwork for circulating air.

HVAC systems can make many different sounds- some are normal and some aren’t—so before you call in professional help, here’s a list of noises and what to do if you hear them.

Common HVAC Noises and How to Address Them

For every HVAC noise, there is a solution (ranging from simple to complex) to silence it. If you’re not sure if what you’re hearing matches a description below, contact a professional.

Squealing or Screeching

This sound can signal a bad fan belt in the blower. If so, the belt needs replacement. The sound may also mean the motor needs a specialized lubricant—the DIY of which is dependent on the make and model of your unit. When in doubt, call an HVAC technician.


If this noise comes and goes with airflow, then it’s indicating something is stuck in the blower’s fan blades. Have the obstruction cleared to eliminate unnecessary wear and tear on the unit.

Rattling (Inside the House), Clanking or Banging

Like “thwapping,” these sounds may be related to the furnace’s blower assembly or motor, and are likely due to something being loose or broken.

If the sound gets louder when the blower is running, shut the system off until you can get help. At this point, something has broken or gotten disconnected.


Popping is common if the heat hasn’t been on for a while and the noise is coming from your ductwork. It’s the sound of cold metal ducts expanding due to the rush of warm air.

If the popping comes from another source or doesn’t go away, contact a technician to uncover the problem.


A buzzing from your AC unit may signal a compressor malfunction, a refrigerant leak causing the unit to freeze, or a loose part vibrating. Contact a technician to examine the system.

Frequent Clicking

If the clicking is coming from the AC unit, it’s often due to the outside compressor or the inside air handler. There are several possible causes, which a technician can diagnose.

Is the clicking coming from the furnace? If the weather is getting colder and you’re turning the heat on for the first time this season, the clicking tells you your system is working.

But if the clicking continues and comes from your furnace, that could be a sign of an ignition system (pilot light or electronic ignition) malfunction. It might get worse—see below.


This happens when the furnace is attempting- unsuccessfully- to light (or relight) from a pilot light or electronic ignition. As it continues, there’s an increasing gas build-up within the combustion chamber that will suddenly ignite all at once.

The problem was likely caused by a build-up of carbon particles. The chamber along with the flue should be inspected and cleaned professionally.

High-Pitched Whistling or Screaming

This is a warning sound. If it’s coming from your AC compressor, quickly turn electrical power to the unit OFF. It might switch off on its own—which is good. This may also signal dangerously high refrigerant pressure or that your compressor is failing. Leave it alone and have an HVAC technician inspect it as soon as possible.


Is the hissing sound coming from the furnace or the AC compressor?

If it’s coming from the AC compressor, there’s may be an internal refrigerant leak. It may also indicate dangerously high internal pressure. You should disconnect the power and call a technician.

If it’s coming from in or around the furnace, it’s likely a leak—which requires professional repair.

Rattling (From the Outside Compressor or Fan Unit)

Rattling may stem from a failing compressor, a pulley with failing bearings or a worn-out compressor clutch. Anyone of these concerns warrants the attention of an HVAC technician.

HVAC systems provide many audio cues about their status. Some sounds point to hardware, while others point to various fluids. Again, an HVAC technician could say for sure what it is.

That’s All … Until We “Hear” More

Your HVAC system maintains a comfortable climate in your home, and we tend to forget it is there until something breaks.

All appliances endure wear and tear and require regular maintenance to continue operating well. Have you had your annual visit from an HVAC technician yet?

If not, contact us for an appointment right away. Otherwise, you might find yourself calling when something breaks at the worst possible moment!

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