AC Making a Strange Noise? What Each Sound Means and What to Do
Buzzing, clicking, hissing, banging, squealing or rattling from the indoor or outdoor unit.
By Mark Curant·Updated
Most likely causes
- Buzzing or humming at the outdoor unit: a failing capacitor or contactor, or a motor that can't start
- Rattling: a loose panel or screw, or debris in the fan
- Squealing or grinding: worn fan or blower motor bearings, or a slipping belt on older blowers
- Hissing: a refrigerant leak (a short hiss right after shut-off is normal)
- Gurgling or bubbling: a partly clogged or dried-out condensate drain
- Banging or clanking: a loose or broken part, or a failing compressor
- Screaming or loud whistling from the outdoor unit: dangerously high pressure. Turn it off now.
Before you open anything
Turn the system off at the thermostat, then switch off power at the breaker and at the disconnect beside the outdoor unit. Never touch the capacitor, even with the power off: it can hold a dangerous charge. Refrigerant work is for licensed technicians only. Reset a tripped breaker once; if it trips again, leave it off.
Never reach into the outdoor fan or remove its grille with the power on. Turn off the disconnect first and wait for the blades to stop completely.
Check these first (free, 5 minutes)
- Find where it comes from: the outdoor unit, the indoor unit (air handler or furnace), the ducts or one vent.
- Notice when: only at start-up or shut-down, the whole time it runs, or only on hot afternoons.
- With power off at the disconnect, look through the outdoor unit's top grille for sticks, leaves or anything touching the fan.
- Check that the filter door and the indoor unit's panels are seated and their screws are tight.
Causes, signs and repair costs
Normal operating sounds
DIYA click at the thermostat or outdoor unit when it starts and stops, a short whoosh or hiss as pressures even out after shut-off, a steady hum while running, ducts ticking as they cool
Typical cost: $0
Failing capacitor
ProOutdoor unit hums or buzzes but the fan doesn't spin, or starts slowly
Typical cost: $150–$400
See: Outdoor unit not running →Failing contactor or loose wiring
ProBuzzing or chattering at the outdoor unit, rapid clicking, sometimes insects nesting in the contactor
Typical cost: $150–$600
Loose panel, screw or debris
DIYRattling or vibration that stops when you press on a panel; sticks or leaves in the fan
Typical cost: $0
Worn outdoor fan motor
ProSquealing, screeching or grinding from the top of the outdoor unit; fan slow to get up to speed
Typical cost: $400–$800
Worn blower motor or belt (indoor)
ProSquealing or grinding from the indoor unit whenever the fan runs; belt-drive blowers on older systems squeal when the belt slips
Typical cost: $75–$200 for a belt, $500–$1,200 for a motor
Refrigerant leak
ProContinuous hissing near the copper lines or coil, cooling getting worse, ice on the lines
Typical cost: $400–$2,400 to find, fix and recharge
Partly clogged or dried-out drain
DIY or proGurgling or bubbling near the indoor unit while it runs
Typical cost: $10–$20 DIY, $75–$200 pro
See: AC leaking water →Ducts flexing or a restricted return
DIY or proPopping or booming in the ducts when the blower starts or stops; whistling at a grille
Typical cost: $0 (new filter, open vents) to duct work
Failing compressor or a broken internal part
ProLoud banging, clanking or knocking from the outdoor unit, often at start-up
Typical cost: $1,500–$3,000
Dangerously high pressure
ProScreaming or a high-pitched whistle from the outdoor unit
Typical cost: Depends on the cause; turn it off first
| Cause | Signs | Who fixes it | Typical cost |
|---|---|---|---|
| Normal operating sounds | A click at the thermostat or outdoor unit when it starts and stops, a short whoosh or hiss as pressures even out after shut-off, a steady hum while running, ducts ticking as they cool | DIY | $0 |
| Failing capacitorSee: Outdoor unit not running → | Outdoor unit hums or buzzes but the fan doesn't spin, or starts slowly | Pro | $150–$400 |
| Failing contactor or loose wiring | Buzzing or chattering at the outdoor unit, rapid clicking, sometimes insects nesting in the contactor | Pro | $150–$600 |
| Loose panel, screw or debris | Rattling or vibration that stops when you press on a panel; sticks or leaves in the fan | DIY | $0 |
| Worn outdoor fan motor | Squealing, screeching or grinding from the top of the outdoor unit; fan slow to get up to speed | Pro | $400–$800 |
| Worn blower motor or belt (indoor) | Squealing or grinding from the indoor unit whenever the fan runs; belt-drive blowers on older systems squeal when the belt slips | Pro | $75–$200 for a belt, $500–$1,200 for a motor |
| Refrigerant leak | Continuous hissing near the copper lines or coil, cooling getting worse, ice on the lines | Pro | $400–$2,400 to find, fix and recharge |
| Partly clogged or dried-out drainSee: AC leaking water → | Gurgling or bubbling near the indoor unit while it runs | DIY or pro | $10–$20 DIY, $75–$200 pro |
| Ducts flexing or a restricted return | Popping or booming in the ducts when the blower starts or stops; whistling at a grille | DIY or pro | $0 (new filter, open vents) to duct work |
| Failing compressor or a broken internal part | Loud banging, clanking or knocking from the outdoor unit, often at start-up | Pro | $1,500–$3,000 |
| Dangerously high pressure | Screaming or a high-pitched whistle from the outdoor unit | Pro | Depends on the cause; turn it off first |
Typical U.S. installed prices in 2026. Most repairs also carry a service call fee of $75–$200, often credited toward the repair. See AC repair costs in 2026 for the full list.
Step by step
- 1
Pin down where and when
Stand by each part of the system while it runs. Note whether the sound comes at start-up, the whole time, or at shut-off. That alone narrows most noises to one or two causes.
- 2
Turn it off for the serious ones
Screaming, grinding, banging, or buzzing while the outdoor fan sits still: switch the system off at the thermostat and the disconnect. Those sounds mean a part is failing, and running it can damage the compressor.
- 3
Clear the outdoor unit
With power off at the disconnect, clear leaves, grass and sticks from around and on top of the unit. Don't reach into the fan or take the top grille off. Keep plants at least 2 feet away.
- 4
Tighten what you can reach
Press on the outdoor unit's side panels and the indoor unit's doors while it runs. If the rattle stops, a panel is loose; tighten its screws with the power off. Make sure the filter is seated and its door closed.
- 5
Record it for the technician
Take a short video with sound, and note when it happens. A technician who hears the sound before arriving can bring the likely part and diagnose it faster.
Call a technician if
- Screaming, grinding, banging or clanking
- Buzzing or humming while the outdoor fan doesn't spin
- Continuous hissing near the refrigerant lines
- Any noise that comes with a tripped breaker or a burning smell
Red flags on a quote
- A new compressor or system quoted for a buzz before anyone has tested the capacitor and contactor
- A refrigerant recharge for a hiss with no leak search
- A fan motor replaced without testing the capacitor that starts it
Facing an expensive repair on an older system? Run it through the repair vs replace calculator, or read the $5,000 rule.
Frequently asked questions
Why is my AC making a buzzing noise?
Most often it's an electrical part at the outdoor unit: a failing capacitor or contactor, or loose wiring. If the fan isn't spinning while it buzzes, turn the system off, because the motor is trying and failing to start and will overheat. Electrical repairs like these typically cost $150–$600.
Is it normal for an AC to click?
A single click when the system starts and stops is normal: it's the thermostat and a relay switching. Rapid or continuous clicking, or a click with nothing starting, points to a failing contactor, relay or control board.
Why is my AC making a hissing sound?
A short hiss or whoosh right after the system shuts off is usually normal, as the refrigerant pressures even out. A continuous hiss near the lines or coil can be a refrigerant leak, especially if cooling is getting worse or ice forms on the lines. Have it checked.
What does it mean when an AC is screaming?
A scream or high-pitched whistle from the outdoor unit can mean dangerously high pressure inside the compressor or a serious refrigerant leak. Turn the system off at the thermostat and the disconnect right away and call a technician.
Related problems
Outdoor unit not running
The indoor fan blows but the outside unit is silent, humming or its fan is still.
AC smells bad
A musty, burning, rotten-egg, sour or chemical smell when the AC runs.
AC short cycling
It runs for a few minutes, shuts off, then starts again soon after.
Fixed it? Check what it costs to run
If your system is old enough to keep breaking, compare its running cost with a new one before the next repair.
Open the SEER savings calculator