HVAC Noises and What They Mean

When your HVAC system is running correctly, you may forget that it is even there. You have become accustomed to the familiar grunt of your a/c kicking on and then the constant hum that means it is working as it should. What happens when that familiar sound is replaced by a screeching or clunking noise? Will you have any idea what is wrong or how to fix it? After reading this article, you should be able to identify at least some of the odd noises that an HVAC system can produce.


Your a/c unit should make a humming noise while running correctly. This sound can vary depending on age, cleanliness, size, and other factors. This will be the standard sound for your a/c and should not cause concern.


If the outdoor unit for your home is making a rattling noise, then there are several things that it could be. For example, it could be caused by debris in your unit, such as a twig, leaf, or other fragments rubbing along your fan blades while spinning. 

Another cause could be a fan shroud issue from an unbalanced fan blade or a loose screw in the shroud. 

The rattling could also come from a loose panel. Your a/c will cause a certain amount of vibration while it is running. If one of the side covers is loose, then it can cause it to make a rattling sound. 


A whoosh sound is commonly heard in heat pump systems, but sometimes a straight a/c system can make similar sounds. When the system switches from heat to a/c or vice versa, the reversing valve in the unit will switch and reverse the flow of refrigerant, which causes a loud whooshing sound.


This sound is rarer than the other ones, but it happens. It will be a very fast and loud chattering. It will emanate from behind the cover, housing your air conditioner’s electrical components. It is your contactor making this noise, and it is what completes the circuit allowing your air conditioner to run. Its chattering will mean dirty contact points, ant infestation, or age. Whatever the reason, this will need to be addressed by a professional.


Unlike the others, this noise is found in the outside and inside units. It is the sound your fan motor will make when the bearings are beginning to seize up. If you watch your air conditioner start, this may be associated with the fan slowly turning before it speeds up or continuing to spin slowly. 

The screeching can also be found in the indoor unit coming from the blower fan. The bearings within your blower fan motor are also likely to wear out and start screeching given enough time. 

If your indoor unit is a much older one, it may have a belt turning the fan blade. These are becoming increasingly rare, but there are still those in use. If you have a fan belt, it may cause a screeching sound like the screeching of a fan belt on your vehicle. 


If you hear a whistling sound, it may be best to try and track it down because there can be multiple reasons. It could be that your air filter in your return grill does not have a good seal or is dirty, so it is sucking the air past the edges, creating a whistling sound.

It could also be coming from a vent, which could mean that the vent has too much air, the vent cover is partially closed, or the cover is not sealed correctly, causing the air to “whistle” past an edge.

Another possibility, it is coming from a hole in the ductwork. If your ductwork has a hole in it, it can cause the air to squeeze through the opening and produce a whistling sound.


Clicking is a sound that will be heard on the inside and outside when the units turn on and off. A single click usually represents a contact situation, where power is making contact to proceed to the next step of the unit’s process. It may be the air conditioner making contact, so it can start, or the indoor unit clicking closed for the fan to begin running. Either way, you will hear a clicking sound followed by the unit making its next step.

However, if the clicking sound continues and does not seem to stop unless the unit is not running, then it may be that something has gotten stuck in the blower wheel of your indoor unit.

Heating Noises

Depending on whether you have a gas furnace or an electric air handler, you will hear different noises while it is heating. If it is an air handler or what may be called an electric furnace, you should be able to listen to the flow of the refrigerant, like water running through a garden hose, while it is heating. If it is freezing outside, to the point that your heat pump cannot keep up, then there may be a buzzing noise that starts inside the unit caused by your heat strips. Metal wires are positioned in the unit to warm the air, similar to a toaster. The wires turn red hot and heat the surrounding area.

If you have a fuel-burning furnace, then you will hear different sounds. First, you will listen to the inducer fan kick on, which could make the squealing sound of bad bearings if it is beginning to go out. Then there will be a click, or a series of clicks, which will be your igniter starting up. Next will be a whoosh, followed by an ignition sound. The furnace will proceed to sound like a jet engine but much quieter until it has reached a sufficient temperature and starts your blower motor.

Final Thoughts

Most of us have a decent idea of the sounds that our HVAC system is supposed to make. Or we are at least aware enough to recognize when it is making some absurd sound that we have never heard before. Odd sounds may mean something is wrong with your HVAC system, and it will need to be looked at by a professional before the problem worsens. Your a/c and furnace are highly technical pieces of equipment and should be serviced by an HVAC professional. Although there may be some things on this list that you can evaluate and repair yourself, we highly recommend that you leave the majority of it to the professionals.