Why Is My Air Conditioner Making A Knocking Noise? [ 4 Causes & Solutions]

In normal conditions, air conditioners, like most household appliances, are quiet. The only sound you should hear is a low, steady blowing noise. If your air conditioner begins to make a loud knocking or banging noise, something is wrong with it.

The knocking noise from AC often comes from the stuck particles in the condenser, loose parts, or a faulty fan or compressor.

Some can be fixed on your own, but others require the services of an HVAC professional.

In the following section, I will explain why these issues can cause such noise in your AC, as well as provide solutions. Let’s get started!

1. Stuck particles in the AC condenser

particles on the condenser

Except for portable models, the condenser is usually located outside the house, regardless of the type of AC you have. Because heat must be transferred outside, the case of the outdoor unit is not completely sealed.

Twigs, leaves, and other large or small particles can get into the unit and become stuck in the condenser over time. When these particles interfere with the condenser’s normal operation, you will hear a knocking sound from the outside unit.

What to do?

Before you begin, turn off the power supply. You can remove the particles by opening the condenser cover. If it is difficult to access, contact a local HAVC professional.

2. Loose components

Your air conditioner requires routine maintenance. It is not only because the machine will collect a lot of dust and dirt, but also because the screws, nuts, piston pins, rods, and crankshaft can become loose due to normal wear and tear.

When these parts become loose, they will strike the part or case around them, causing a knocking or banging noise.

What to do?

Locate where the noise comes from, turn off the power supply, and then try to tighten these parts. 

3. Malfunctioning fan

malfunctioning fan

A fan can fail due to a variety of factors. For instance, accumulated debris can cause the fan motor to become unbalanced. This will result in some fan blades colliding with the case and its components.

Furthermore, when the fan is loose, the blades make loud noises.

A faulty blower frequently begins with a clicking noise and progresses to a knocking noise as the problem worsens.

What to do?

It is sometimes possible to solve the problem by removing the buildups on the fan motor and fan blades. Tightening it with a screwdriver may also work. If not, you should hire a professional to repair or replace it.

4. Broken Compressor

This is the last thing you want to hear, but in rare cases, the knocking noise could be caused by a faulty compressor. When some parts of the compressor wear out, they will collide with other parts of the compressor, causing loud noises.

What to do?

The compressor is the core part of an air conditioning unit, which involves complex mechanisms. Accordingly, it is expensive to repair or replace. 

As a result, if your AC has been in service for several years, you should consider purchasing a new unit.

How to prevent the knocking noise from coming back?

While wear and tear are unavoidable when running an air conditioner, you can reduce knocking noise by committing to regular cleaning and maintenance.

This way, the particles will be removed from the outdoor unit, and the debris on the fan or blower will not affect the spinning of fan blades. 

Aside from that, HVAC professionals will inspect the condition of each component of your air conditioner and tighten them as needed.


If you hear your AC making a knocking noise, don’t leave it unattended because it can result in more serious problems sooner or later.

A knocking noise will arise when components hit the case or other parts. So you should check if the fan blades, screws, or other parts are loose. The particles in the outdoor unit could also be the culprit.

If none of these apply, it could be due to a faulty compressor. You may be able to get a replacement if your machine is still under warranty. Otherwise, repairing a compressor could be prohibitively expensive.

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Liz Yang is the founder of Airsmartly. She has been working at home for a few years and realizes that the performance of the HVAC system plays such an important role in our life. She has tested a lot of products in person, like humidifiers, air purifiers, dehumidifiers, and ACs, and wants to share tips about using or troubleshooting these products with you. Her uncle is an HVAC expert with over 30 years of experience in the field, and often offers assistance when she is unsure how to handle a situation. He is also in charge of reviewing the articles on this site.