Why is My Outside AC Unit Making a Loud Noise?

Strange noises coming from your air conditioner are never good. You’re probably wondering if the noise means there’s a serious problem with your AC system.

Note: If you hear any strange noises coming from your AC, we recommend turning off your system and calling a professional immediately. Letting your AC run could seriously damage your system or harm your home.

Unfortunately, without inspecting your condenser (your outside unit), there’s no way of knowing exactly what’s wrong with it.

But to give you an idea of what could be the problem, we’ll go over the following noises and what they could mean for your AC:

• Screeching
• Slamming/banging
• Screaming/whistling
• Buzzing

We’ll also let you know whether or not you should turn off your AC for each noise.

Complaint: When your AC is running, you hear a loud, metal-on-metal screeching noise that won’t stop until the AC shuts off.

Probable cause: Your fan motor bearings are bad, which means you’ll have to replace the fan motor.

Complaint: Your outdoor unit makes a loud slamming noise when it shuts off or a banging noise while it’s running.

Probable causes:

• Does the noise happen when your unit turns off and/or on? Your compressor’s internal springs are probably shot. Inside your compressor (the part that pumps refrigerant through your AC) are pumps mounted on support springs. When a spring breaks, the pump isn’t held evenly anymore. When the pump is operating, it has enough momentum to maintain its course, but when your AC turns on/off, the pump will lose that momentum, start to wobble and knock against the case of the compressor, creating a banging/slamming noise.
• Does the noise happen while your unit is running? The problem is likely the fan. Parts on condenser fans can come loose and knock against other parts of the outdoor unit, making a banging noise.

Complaint: You hear a high-pitched whistling or screaming sound for 10–15 minutes after the AC first kicks on. Your AC might also turn on and off frequently.

Probable cause: Your compressor has high refrigerant pressure. Some AC units have a sensor that shuts down the system if it detects dangerous levels of internal pressure, which explains why your AC might be turning on and off a lot.

Complaint: You hear a constant buzzing noise in your condenser, both when the unit’s running and when it’s not.

Probable cause: There’s an electrical problem in your system. The problem could be caused by the:

• Contactor relay switch: This is a low-voltage switch located inside your condenser, but controlled by the thermostat. If the switch or thermostat is faulty, you might hear a buzzing noise.
• Condenser fan motor: If your outdoor fan isn’t turning when the inside blower is working, you’ll hear a buzzing noise in your condenser.
• Electrical connections between parts: Electrical connections that are old or coming loose can cause electrical arcing, which creates a buzzing/humming noise.