Why Is My Air Conditioner Making a Knocking Noise?

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Updated May 18, 2023

Air conditioners produce noise levels by nature, but there are certain sounds to listen for that may indicate problems and a need for repair. The best air conditioners should run without knocking, banging, or clattering noises. While this noise could be caused by something as simple as debris or a dirty air filter, it may also be a compressor issue, so it’s best to have your unit checked by an HVAC professional. Below we’ll explain the causes of such noises for anyone asking themselves, “Why is my air conditioner making a knocking noise?”


  • If you hear your air conditioner knocking, it is best to turn it off and call a professional to look at it.
  • The most common cause of knocking noise is loose or damaged compressor parts.
  • Always keep your filter and outdoor compressor area clean and free of debris and obstructions. Failing to do this can damage parts, resulting in knocking sounds.

Additionally, We have similar content covering other topics, such as why your air conditioner makes a loud noise when turning it on.

Insider Tip

As soon as you hear a knocking or loud noise, turn your air conditioning unit off and call a technician to come and diagnose your issue.

Why Your Air Conditioner Makes a Knocking Noise

An outdoor unit that makes knocking noises can point to many things, ranging from easy maintenance measures to severe damage. While it’s best to call a professional to examine your unit, an excellent first step is to investigate where the unusual noises might be coming from. Something that can be handy in discovering the cause of the strange sounds is knowing what types of AC motors can be used in your air conditioner.

And suppose you hear higher-pitched sounds coming from your unit. In that case, you can read our article explaining why your air conditioner is squealing.

Broken or Damaged Compressor

A compressor issue is the most common culprit of strange knocking or rattling noises. Any loose components, like a fan blade, can create a strange noise. Refrigerant leaks can also be a source to trigger the knocking or clicking noises in AC systems. Usually, this is a minor issue, resolved by a quick tightening. However, it can severely damage parts if left unresolved, requiring a full compressor replacement. It’s important that if you catch this air conditioner noise during your regular maintenance checks, you should address it as soon as possible in order to mitigate the issue before it grows larger and you wind up needing expensive repairs.


Sometimes leaves, nuts, dirt, sticks, or rocks can get inside the unit and cause AC compressor noise. The debris can get stuck and rattle around, making a loud noise.


Make sure you empty the condensate tank regularly, or you might end up with mold inside your AC unit.

Dirty Air Filter

Failing to clean or replace your air filter causes unwanted stress on the blower motor. The blower is a separate fan that pushes air through the ducts of a house. When stress in the blower motor builds up, it can eventually produce knocking sounds.

STAT: By changing your air filter regularly (every 30-90 days), homeowners can save anywhere from 5-15% on monthly electricity bills. (source)

Why Is My Air Conditioner Making a Knocking Noise FAQs

At what point should I replace my noisy air conditioner?

If your unit is old and experiencing problems and you are getting unusually high electrical bills, it may be worth purchasing a new air conditioner.

How can you reduce air conditioner noise?

You can remove the side panel to the compressor and remove any visible debris blocking it from getting in the way of the fan blades. You can also install a sound blanket over your compressor.

What does it mean if my AC unit makes a hissing sound?

Hissing typically indicates a refrigerant leak, which can be dangerous and needs to be fixed as soon as possible. If you hear a hissing sound, turn off your AC immediately.
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