A well-functioning AC operates quietly in the background. When unusual sounds emerge, your system is communicating that something needs attention.
Normal AC Sounds
First, know what's normal:
Gentle humming when compressor runs
Soft whoosh of air through vents
Click when system starts and stops
Occasional drip of condensation
These sounds indicate normal operation.
Buzzing Sounds
Possible Causes:
Loose parts vibrating
Debris in outdoor unit
Failing contactor
Refrigerant leak
Electrical problems
What to Do:
Check outdoor unit for visible debris
Listen for whether sound is inside or outside
Electrical buzzing requires professional attention
Don't ignore persistent buzzing
Urgency Level: Moderate to High
Clicking Sounds
Normal Clicking:
Single click when system starts/stops
Thermostat switching modes
Relay engaging
Abnormal Clicking:
Repeated clicking without starting
Clicking throughout operation
Rapid clicking sounds
Possible Causes:
Failing thermostat
Control board problems
Compressor issues
Capacitor failure
Urgency Level: Moderate (prevents startup) to Low (during operation)
Rattling and Banging
Rattling:
Loose screws or panels
Debris in unit
Fan blade hitting something
Ductwork issues
Banging:
Loose or broken part inside compressor
Unbalanced blower wheel
Motor mount failure
Major component failure
What to Do:
Turn off system if banging is severe
Check for visible loose components
Banging often indicates serious problems
Urgency Level: High for banging, Moderate for rattling
Squealing and Screeching
Possible Causes:
Worn fan belt (older systems)
Bearing failure in motor
High internal pressure
Metal-on-metal contact
What to Do:
Turn off system immediately if screeching
Don't operate until inspected
High-pitched squealing can indicate dangerous pressure
Urgency Level: High
Hissing Sounds
Possible Causes:
Refrigerant leak (most common)
Leaking air ducts
Internal valve problems
High compressor pressure
What to Do:
Hissing from refrigerant lines needs immediate attention
Check visible ductwork for disconnections
Refrigerant leaks require professional repair
Urgency Level: High for refrigerant leaks
Gurgling or Bubbling
Possible Causes:
Refrigerant leak or low charge
Condensate drain problems
Air in refrigerant lines
What to Do:
Check condensate drain for clogs
Refrigerant issues require professional service
Bubbling often indicates moisture in the system
Urgency Level: Moderate
Humming That Seems Louder Than Usual
Possible Causes:
Failing compressor
Electrical problems
Capacitor issues
Motor problems
What to Do:
Note when humming occurs
Check if system is cooling properly
Increasing hum often precedes failure
Urgency Level: Moderate
When to Call Immediately
Don't wait if you experience:
Any grinding sounds
Screaming or high-pitched squealing
Loud banging from compressor
Electrical buzzing with burning smell
System won't stop making noise
Diagnostic Tips
Before calling for service:
Note exactly when the sound occurs
Identify whether sound is from indoor or outdoor unit
Record video of the sound if possible
Note if sound correlates with certain functions
This information helps technicians diagnose problems faster and more accurately, potentially saving you time and money on the service call.