Loud humming with no cooling — stuck capacitor

Sound: Steady, loud electrical hum from outdoor unit. Fan not spinning, or spinning very slowly.
What it means: The dual-run capacitor that starts the compressor and condenser fan has failed. The motor is trying to start and drawing power, but can’t move.
Urgency: Shut the system off immediately. Continuing to run it for more than 10 minutes can burn out the compressor — a $1,800–$3,500 repair vs a $200 capacitor.
Cost to fix: $180–$280, parts and labor, takes 30 minutes.

Clicking from outdoor unit, won’t start — relay or contactor

Sound: Repeated click-click-click from outdoor unit when system calls for cooling. No fan or compressor running.
What it means: Contactor (the big relay that powers the compressor) is failing. Common after Florida lightning storms.
Urgency: Shut off, schedule repair within 24 hours.
Cost: $200–$300.

Screaming or hissing — refrigerant leak or compressor pressure

Sound: High-pitched scream or hiss, sometimes only when starting up.
What it means: Either refrigerant leaking under pressure, or compressor internal pressure is dangerously high.
Urgency: Shut off NOW. Pressure issues can rupture lines or seize the compressor.
Cost: $300 (pinhole leak) to $2,500 (compressor replacement).

Banging or clanking — broken or loose part

Sound: Sharp metallic bang or clank, intermittent.
What it means: Usually a fan blade hitting something (a stick, a piece of broken plastic shroud), or a piston/rod inside the compressor that’s coming loose.
Urgency: Shut off, look outside. If a stick is in the fan, remove it and check for blade damage. If you don’t see anything obvious — call a tech, internal compressor damage gets worse fast.
Cost: $0 (stick removal) to compressor replacement.

Squealing or grinding from indoor unit — blower motor bearings

Sound: High squeal or low grind from the air handler.
What it means: Blower motor bearings are failing. Old PSC motors can run a long time after this starts; modern ECM motors fail completely soon after.
Urgency: Schedule repair within a week. If it stops moving entirely, you’ll have no airflow and the system shuts off via safety switch.
Cost: $400–$700 motor replacement, $1,000+ if ECM control module also failed.

Bubbling or gurgling — refrigerant low or condensate trap issue

Sound: Low bubbling or gurgle at the indoor coil or in lineset.
What it means: Either refrigerant is low (you’re hearing two-phase flow), or condensate water is moving the wrong way through a trap.
Urgency: Run the troubleshooting steps from our cooling guide. If cooling is normal, it’s likely a condensate issue and harmless. If cooling is weak, schedule a leak check.

Rattling — loose panel or ductwork

Sound: Vibrating rattle, often only at certain blower speeds.
What it means: A panel screw is loose, or sheet metal duct is vibrating against something.
Urgency: Annoying, not urgent. Check service panel screws first; tighten anything loose.
Cost: Free if you DIY; $80–$150 for a tech to track it down.

Buzzing from indoor wall — bad transformer

Sound: Faint 60Hz buzz from the air handler, even when system is off.
What it means: Low-voltage transformer (24V) is failing or shorted.
Urgency: Schedule within a week. If it fails completely, no cooling.
Cost: $150–$220.

FAQ

How loud should an outdoor AC unit be normally?
Modern (post-2020) units run 50–62 dB at 3 ft — about as loud as conversation at a restaurant. Older or premium units (variable speed) can run as low as 45 dB. Anything above 70 dB is a problem.

Can my neighbor’s AC be making noise that I think is mine?
In Boca’s zero-lot-line communities, yes. Walk to your unit and confirm the sound is coming from yours. Florida noise ordinances generally allow up to 65 dB during day hours at the property line.

Need it diagnosed today?

AtlantFlow runs same-day service across Palm Beach, Broward, and Miami-Dade. Licensed FL CAC1824422.

Call (561) 510-9414