Cooling Emergency
Strange Noises from Your AC?
Banging, buzzing, squealing, hissing — your AC is trying to tell you something. Here's how to decode it.
What's Going On
Understanding the Problem
Your AC system has started making an unusual noise — something it didn't do before. AC systems have multiple moving parts (compressor, fan motors, blower wheel, relays) and different noises point to different problems. Some are minor, some mean "shut it off right now."
Diagnosis
Common Causes
1Banging or clanking
A loose or broken part inside the compressor, a disconnected blower fan blade, or a part that's come loose and is bouncing around inside the unit. This is usually serious — stop the system and call.
2Buzzing
Electrical buzzing often means a failing contactor, loose wiring, or a dying compressor. Outdoor unit buzzing without the fan spinning usually means the capacitor has failed. Moderate urgency.
3Squealing or screeching
A belt slipping (in older systems) or a fan motor bearing going bad. The motor is struggling and will eventually seize if not addressed. Not an immediate emergency, but don't ignore it.
4Hissing
A hissing sound from the refrigerant lines or indoor unit often indicates a refrigerant leak. If it's loud, it's a significant leak. This is an urgent call — running the system low on refrigerant damages the compressor.
5Clicking (repeated)
Constant clicking from the outdoor unit means the system is trying to start but can't. Usually a failing relay, contactor, or capacitor. Single clicks when the system starts and stops are normal.
6Rattling
Loose screws on panels, debris inside the outdoor unit (sticks, leaves), or ductwork that's come loose. Usually not urgent but worth investigating before it becomes something worse.
DIY Troubleshooting
What You Can Try
Identify the source
Is the noise coming from the outdoor unit, the indoor air handler, or the ductwork? Knowing where narrows down the cause significantly.
Check for debris in the outdoor unit
Turn the system off and look through the fan grate on top of the outdoor unit. Sticks, leaves, or small objects can fall in and hit the fan blade, causing rattling or banging.
Tighten loose panels
If it's a rattling noise, check the access panels on the outdoor and indoor units. A loose screw can make a panel vibrate and create noise that sounds alarming but is actually harmless.
Shut it off if it sounds serious
If you hear grinding, metal-on-metal, or loud banging — turn the system off at the thermostat immediately. Running it will cause more damage.
Know When to Call
When to Call a Pro
Banging, hissing, grinding, or metal-on-metal sounds warrant an immediate call. Buzzing and clicking mean you should call soon — within the day. Squealing and rattling are less urgent but should be addressed within a week before they escalate.
Pro Tip
If your outdoor unit makes a loud buzzing noise and the fan isn't spinning, try this (carefully): with the system on, take a long stick and gently push one of the fan blades to give it a start. If the fan starts spinning and runs normally, the capacitor is failing — it can't give the motor enough juice to start, but once spinning, the motor can sustain itself. Call for a capacitor replacement — it's a quick, inexpensive fix.
More Troubleshooting