Why Is My Air Conditioner Making a Loud Noise?

AC units are not silent, but they should operate at a consistent, low hum. A new or sudden noise - banging, squealing, clicking, rattling - is the system telling you something is wrong. Here's what different sounds usually mean.

Banging or Clanging

Most likely cause: A loose or broken component inside the compressor (indoor part hitting the housing), or a broken fan blade on the outdoor unit hitting something.

What to do: Shut the system off. Running it with this noise can cause additional damage quickly. Call a technician.

Severity: High. This is not a maintenance issue - it's an active failure.

Squealing or Screaming

High-pitched squeal or screech:

• Outdoor unit: Can indicate refrigerant pressure is dangerously high. Older units sometimes have a pressure relief valve that produces this sound. Shut the system off immediately if the sound is sudden and loud.
• Indoor unit/air handler: Often a worn blower motor belt (older units with belt-driven blowers) or a motor bearing failing.

Belt squeal: Some older air handlers use a belt-driven blower. If the belt is worn, it squeals. Replacement is relatively inexpensive ($100-$200) and should be done quickly before the belt fails.

What to do: If the sound is from the outdoor unit and sudden, shut off and call immediately. Blower belt squeal is less urgent but schedule a repair within a few days.

Clicking (Repeated)

At startup only: Normal. The system clicks once when the contactor engages and the compressor starts.

Repeated clicking without the system starting: Likely a failed capacitor or a relay issue. The system is trying to start but can't.

Clicking during operation: Electrical relay issue or failing contactor. The contactor opens and closes to control the compressor - clicking during operation suggests it's struggling.

What to do: A capacitor or contactor is a common, relatively inexpensive repair ($150-$350). Schedule with a technician.

Rattling

Indoor unit: Loose screws or panels, debris in the air handler, or a loose component in the ductwork near the air handler. Check for and tighten any loose covers on the air handler.

Outdoor unit: Debris (leaves, small sticks) inside the unit hitting the fan blade. Turn off power, look inside the unit (remove the top if accessible), and remove any debris. Also check that the unit is level - a slightly unlevel unit can vibrate and rattle.

In the ductwork: Air duct rattles are usually loose duct connections or flex duct that's vibrating. Less urgent, but worth sealing to prevent energy loss.

Buzzing

Steady low buzz from outdoor unit: Often an electrical issue - loose wiring, failing capacitor, or a contactor that's starting to fail. Can also be a refrigerant leak creating a hissing/buzzing sound.

From the air handler: Blower motor starting to fail, or electrical issue at the relay.

From the ductwork: Duct vibration, or air whistling through a gap.

What to do: Electrical buzzing should be diagnosed by a technician. A refrigerant-related buzz needs immediate attention.

Hissing

Steady hissing from refrigerant lines or unit: This is the sound of refrigerant escaping. A refrigerant leak is an environmental issue and needs to be repaired by a licensed technician (EPA 608 certified). Don't run the system with a known refrigerant leak - you'll damage the compressor.

Hissing when the system turns on/off: Can be normal (refrigerant pressure equalizing). If it's brief and only at startup or shutdown, it's usually not a concern.

Humming (Outdoor Unit Won't Start)

If the outdoor unit hums but doesn't start, the compressor is trying to start but can't. Most common cause: failed capacitor. Secondary cause: the compressor itself is failing.

A capacitor replacement is fast and inexpensive. Have a tech diagnose before assuming compressor failure.

What to Tell the Technician

When you call for service, be specific: - What is the sound? (Bang, squeal, click, rattle, buzz) - Is it from the indoor unit, outdoor unit, or ductwork? - When does it happen? (Startup, during operation, shutdown, constantly) - Is the system cooling normally or not?

This information helps the technician bring the right tools and parts on the first visit.

Bottom Line

Most AC noises mean something specific. Banging and screaming require immediate shutdown. Clicking, buzzing, and rattling range from minor (debris) to moderate (capacitor). Hissing refrigerant leaks need prompt repair. When in doubt, shut the system off and call a tech - running a damaged system causes more damage.

Sources and editorial notes

Prices and permit requirements vary by location, property, equipment, and scope. Treat price ranges as planning estimates and confirm current requirements with the relevant authority and contractor.

• California Contractors State License Board - when a contractor license is required
• US Department of Energy - Energy Saver
• US Consumer Product Safety Commission - home safety

Related guides

• Water Heater Making Noise? What Each Sound Means
• HVAC Repair Cost: What to Expect and How to Avoid Overpaying
• Cost to Replace an Air Conditioner in California

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