Most HVAC failures announce themselves days or weeks in advance with a noise. Here's the same sound-to-cause list our techs run in their heads on a diagnostic call, so you can describe it accurately on the phone — and know whether it can wait until morning.
Noises that mean shut it off now
- Loud metallic banging or clanging: a loose blower wheel or broken fan blade striking the housing
- Continuous grinding: bearing failure in the blower or condenser motor
- Loud electrical buzzing with a burning smell: arcing at the contactor or control board
Noises that mean schedule a visit this week
- Squealing on startup: worn blower belt or dry motor bearings
- Clicking that repeats but the system never starts: failing capacitor or contactor
- Rattling panels: loose screws or a vibrating cabinet — cheap fix, but it wears mounts
- Hissing near the indoor coil: a refrigerant leak (also causes weak cooling)
Noises that are usually normal
- A single click at the start and end of each cycle (the relay)
- Soft whooshing airflow
- Popping or ticking in the ducts as sheet metal expands and contracts
How to describe it so we bring the right part
Tell us three things: when the noise happens (startup, running, shutdown), where it comes from (indoor unit, outdoor unit, vents), and whether the system still heats or cools. That's usually enough for our tech to load the likely part on the truck before rolling out.
Frequently asked questions
My AC clicks but won't start — what is it?+
Most often a failed run capacitor or contactor. It's a common, affordable repair, but running the unit in that state can burn out the compressor.
Is duct popping a problem?+
Usually just thermal expansion. If it's loud and repeated, oversized ducts or high static pressure may be to blame.
Why does my furnace squeal on startup?+
Dry bearings or a worn belt on the blower motor. It won't fix itself and eventually seizes the motor.
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