A noisy indoor unit can be caused by a loose, broken, or out of balance blower wheel.

Noisy indoor unit complaints usually fall into one of six different categories:

1.Loud scraping, metal hitting metal sound

2.Loud thumping or vibrating noise

3.Loud humming noise

4.Rattling, squealing, or high-pitched sound

5.Loud pop or bang sound


1. Loud scraping, metal hitting metal sound- First of all, the customer should turn the system off immediately. This is usually the worst of the six different sounds. One cause for this sound is that the blower wheel came loose from the motor shaft, moved, and is hitting the blower housing. In this case, if no damage was done to the wheel or the motor shaft, it could be moved back to the proper spot and re-tightened to the motor.

Another and more likely cause is that the blower wheel actually broke, in which case it would need to be replaced. This makes a very disturbing sound almost as bad as fingernails on a chalkboard.

A third possibility is that a motor mount broke, causing the entire motor and blower assembly to drop and the blower wheel is hitting the housing. In any case, the system should be shut off as soon as possible when this sound is heard or the repair bill could significantly increase.

2. Loud thumping or vibrating noise- Sometimes this sounds like an out of balance washing machine. This is usually a blower wheel out of balance. Sometimes the motor itself can become out of balance as well. The sound isn't usually too bad but shouldn't be ignored because it can cause other problems to the system.

3. Loud humming noise- If the unit is working fine then most likely it is just a noisy transformer. Some transformers are louder than others and they all hum to some degree.

If the unit is not working and a hum is heard it could be a bad indoor fan motor and/or capacitor.

4. Rattling, squealing, or high-pitched sound- This is for heat pumps only. This is usually refrigerant-flow related, such as a check valve. It is not always a problem. Some units have noisy check valves or metering devices. If the sound gets louder or annoying and the air filter has been checked then a service call should be placed.

5. Loud pop or bang sound- This usually happens when the system fan first starts and/or stops. This is also only found on systems with sheet metal ductwork and it is usually the return trunk.

When the fan first starts the duct "pops in" from the negative pressure of the fan. When the fan shuts off, the pressure releases and the duct "pops out."

This could be as sign of undersized duct, a clogged filter, closed vents, or flimsy duct requiring bracing.

6. Rumbling- This applies to a gas or oil-fired system. If the customer hears an unusual rumbling sound associated with combustion from a gas or oil-fired appliance, this indicates what could be a serious problem with the burner portion of the appliance. The customer should turn off the equipment if necessary.

So to sum it all up, below is a list of possible causes and items to check:

• Loose blower wheel

• Broken blower wheel

• Out of balance blower wheel

• Broken motor mount

• Bad motor bearings

• Noisy transformer

• Noisy refrigerant check valve

• Sheet metal duct popping

• Faulty combustion

Remember - these are just rough guidelines and not all possible situations are covered.

Reprinted with permission from Hannabery HVAC from the company’s Web page “Commonly Reported HVAC Problems.” For more information, visit www.hannabery.com.

Publication date:07/09/2007