Why Is My Air Conditioner Making a Knocking Noise?

There’s nothing quite like that peaceful moment when your AC kicks on and starts cooling your home on a hot, muggy day – until bang! A loud, jarring noise cuts through the silence. If your air conditioner is making a knocking noise, it’s not something to ignore. That sound is your system’s way of telling you something’s wrong.

Let’s break down what might be causing the AC knocking or banging sound, what it means, and how to fix it.

What Kind of Bang Are We Talking About?

Not all AC noises are the same. If you’re hearing a sharp, metallic “bang” – like something slamming or knocking around inside – it likely means something is loose, broken, or out of alignment.

This isn’t the normal hum of a healthy unit. Banging usually signals mechanical trouble that could get worse over time and lead to expensive repair.

Common Causes of an AC Knocking Noise

1. Loose or Broken Parts in the Compressor

Your AC’s compressor is the heart of the system, and when something goes wrong in there, you’ll hear about it. Over time, internal parts can break or come loose. When the unit turns on or off, these parts can clank or bang around. This is more common in older units or those that haven’t had regular maintenance.

If the compressor is damaged internally, a repair might not be possible. Replacing the entire compressor – or even the AC unit – could be necessary.

2. Fan Blade Issues

If the outdoor condenser fan gets loose or bent, it may hit the inside of the unit when spinning. This contact can create a rhythmic banging or clanging sound. In some cases, debris like sticks or rocks can also get inside and cause similar noise.

Check the fan visually (when the unit is off) and see if anything looks bent or out of place.

3. Loose Motor Mounts or Blower Assembly

Inside your indoor air handler or furnace, the blower motor runs the fan that circulates air. If the blower motor’s components, such as the blower wheel or bearings, become worn or loose, it can cause the moving parts to shift and bang against nearby parts.

You might notice this knocking or banging sound coming from inside your home, especially when the system starts up.

4. Damaged Ductwork

Your home’s ductwork can also be the source of a knocking noise from your AC. This often happens when ducts expand or contract with temperature changes, creating a loud “oil canning” sound, especially if the sheet metal is thin or poorly secured.

If the knocking is coming from inside the walls or ceilings, have an HVAC technician inspect the ductwork for pressure imbalances, loose sections, or the need for reinforcements like braces or dampers.

When to Call a Professional

If you hear a knocking or banging noise once, it could just be a one-time issue, maybe a small branch hit the unit or something shifted slightly. But if the sound repeats, don’t ignore it.

You should call an HVAC technician if:

• The knocking gets louder or happens every time the AC runs
• Your system turns off shortly after turning on
• Airflow is weak or inconsistent
• You see visible damage or loose parts

Letting the system run with a mechanical issue can cause more damage – and a bigger repair bill down the road.

How to Prevent Knocking Noises in the Future

Like most HVAC problems, prevention starts with regular maintenance. A seasonal tune-up helps catch loose parts, worn components, or early signs of damage before they turn into a loud surprise.

Here are a few quick tips:

• Schedule HVAC maintenance in the spring and fall
• Keep the area around your outdoor unit clear
• Listen for unusual noises and report them early