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Why does my air conditioner make a loud noise when starting?

AC condenser unit

Loud, surprising noises are the last thing you want to hear from your home. We come to expect certain sounds from our air conditioning units, refrigerators, dishwashers, clothes dryers, and other appliances, but a new and unexpected noise can be a sign that something is wrong.

A loud noise when your air conditioner starts up could be a normal part of its operation, but it could mean something needs to be repaired.

Potential causes of an AC unit making a loud noise when starting

Metal ducts expanding/contracting with temperature changes

The sudden rush of cold air from your air conditioning unit causes a temperature change in the ducts of your HVAC system. Metal ducts are prone to contracting or expanding when the temperature changes, which can cause loud creaking or popping sounds.

Dampers opening and closing

An HVAC damper is a device built into the ducts that controls the flow of air throughout the system. Dampers are handy for temperature zoning (making sure that the right rooms are being warmed or cooled). 

Depending on your system, the loud start-up noise you’re hearing from your AC might be a metal damper banging into the duct as it opens or shuts.

AC compressor issues

The compressor is one of the most important parts of an air conditioning unit. It works to pump the refrigerant from your outdoor unit into your home to do the actual cooling. It’s normal for compressors to make some noises (such as a humming or blowing noise), but an abnormally loud noise on startup may be due to your compressor springs not doing their job.

Fan banging against other parts of condenser

The part of your AC system that sits outside is called the condenser. The condenser relies on a fan to move air throughout your home. If that fan has come loose or become damaged, it might make noise when it collides with other parts inside the condenser. Depending on the damage, this noise might be louder or more likely when the system starts up.

Loose air filter moves when airflow starts

This cause of AC startup noise is fairly harmless compared to some of the other potential diagnoses above. If your indoor air filter is loose, it can move around when airflow starts, resulting in a banging noise.

This issue is also relatively easy to fix!

How to fix loud noises when AC turns on

Adjust or replace your air filter

If you suspect that the noise you’re dealing with is due to a loose or old air filter, then all you need to do is adjust the filter so it’s snug in its spot or replace it with a new filter.

Contact your local HVAC professional

Many of the causes of loud noises when your AC system turns on may require a professional. Unless the noise is caused by your air filter, we do not recommend trying to repair any of the other issues we’ve listed here

Leave it to the professionals! If you live in the Kansas City metro area, you can call A.B. May right now at 913-370-4898 or contact us online to schedule service.