HVAC Basics: Air Conditioner Noise

Air conditioner noise is a common reason that customers call in HVAC repair technicians. Often, the noise itself is enough for an experienced repair tech to make a diagnosis. But if you’re just starting out as an HVAC technician, you might not be intimately familiar with all those noises and what they mean. 

Here’s a primer on common air conditioner noises and the likely causes.

6 types of air conditioner noise and what they mean

1. Whistle and whine

If your customer describes the noise as a whistle or a whine, that’s a pretty reliable indication of an airflow problem. The sound is likely caused by unbalanced air pressure or an air leak or obstruction somewhere. 

Possible causes for this air conditioner noise:

• Damaged ducts leaking air
• Inadequate air distribution (small ducts and a high capacity unit)
• Fan motor speed needs to be adjusted
• Clogged filter

Identifying where the sound is coming from can help you narrow down the cause of the problem.

2. Squeaking and squealing

This mechanical sound is likely caused by parts that are failing or need maintenance. 

Possible causes for this air conditioner noise:

• A blower motor belt may have slipped loose or may be worn
• Motor drive shaft bearings need oiling

A quick check of these components should reveal the cause, and it will likely be a quick and easy fix. Remind the customer that regular maintenance will prevent problems like this!

3. Shaking and rattling

Something is loose that shouldn’t be! In this case of noises caused by vibration, you need to seek out the location of the shaking and rattling to determine what’s loose.

Possible causes for this air conditioner noise:

• If the noise is coming from the ducts, the duct itself might need to be secured, or something like a piece of hardware may be loose inside the duct.
• If the unit itself is shaking, it might need to be secured or stabilized on its base.
• If the rattle or hum is coming from the blower motor, you might find worn bearings or fan blades that are loose.

Once you find what’s loose, this is often an easy fix. Usually you just need to tighten something.

4. Buzzing 

A buzzing sound usually indicates an electrical problem. (Or a beehive in the ducts. It has been known to happen!) 

Possible causes for this air conditioner noise:

• Failed fan motor
• Contactor relay switch needs replacing
• Old wiring or electrical connections
• Circuit breaker failure

Once you find where the buzzing is coming from, your handy multimeter will help you confirm the source of the problem. Note that the problem might not be the AC system itself, but the circuit breaker. (If it’s coming from the ducts, call an exterminator!)

5. Hissing

A low hiss coming from the coils is a sign of a refrigerant leak. In this case, there will usually be additional symptoms such as insufficient cooling. 

You’ll need leak detection equipment to find and fix the source of the leak. In the worst case, a badly damaged coil might need to be replaced.

6. Always running (constant air conditioner noise)

When customers complain that their air conditioner is making noise, sometimes it’s the same sound it normally makes. The customer takes notice because the noise never stops. The air conditioner is running 24/7. That happens when the air conditioner is having trouble reaching the set temperature. 

Possible causes for this air conditioner noise:

• Dirty or frozen coils
• Refrigerant leak
• Blower motor or fan problem
• Faulty controls
• Leaky ducts

In this case, you’ll need to look for more symptoms to pinpoint the cause of the problem. For example, if the system is blowing warm air, that may indicate frozen coils or a refrigerant leak.

More HVAC basics for new (and aspiring!) technicians

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