Most HVAC noises map to a short list of causes: buzzing is usually electrical (a failing capacitor or contactor, loose wiring, or a compressor trying to start), humming without starting is a stuck motor or capacitor, clicking that doesn’t lead to startup is a control or contactor problem, banging and rattling are loose panels, debris or a failing blower, grinding and screeching are bearings and belts and mean shut it off, hissing is refrigerant or a duct leak, gurgling is refrigerant or a clogged drain, and whistling is airflow squeezed through a restriction. Here’s each sound, its likely cause, and how urgent it is.
Sound by sound
| Sound | Where | Likely cause | Urgency |
|---|---|---|---|
| Buzzing | Outdoor unit | Failing capacitor or contactor; loose wiring; compressor struggling; debris hitting fan | Soon; if the fan isn’t spinning, turn off and call |
| Humming, won’t start | Outdoor unit | Capacitor; seized fan or compressor | Turn off; call |
| Clicking, no startup | Thermostat / outdoor unit | Thermostat, control board or contactor; low voltage | Soon |
| Rapid clicking / short cycling | Anywhere | Control, thermostat or safety trips | Soon |
| Banging or thumping | Outdoor unit | Loose fan blade, debris, mounting; compressor internal (serious) | Turn off; call |
| Banging at startup | Furnace / ducts | Delayed ignition (furnace — service soon), duct expansion “oil-canning” (harmless) | Furnace: soon |
| Rattling | Panels, ducts | Loose screws or panels; debris; loose duct | Low; tighten |
| Grinding / metal-on-metal | Blower or outdoor fan | Motor bearings; something in the fan | Turn off now; call |
| Squealing / screeching | Blower / outdoor unit | Belt (older units) or motor bearings; compressor pressure (dangerous) | Turn off; call |
| Hissing | Indoor or outdoor unit / walls | Refrigerant leak; TXV; duct leak; normal defrost on heat pumps (brief) | If continuous: call |
| Gurgling / bubbling | Indoor unit / lines | Refrigerant low or drain issue | Soon |
| Whistling | Returns / vents | Restricted airflow: dirty filter, closed vents, undersized return | Low; check filter |
| Whoosh + hiss, winter | Heat pump | Reversing valve into defrost | Normal |
| Water dripping/trickling | Indoor unit | Condensate; check drain if constant | Low; see leaking water |
What to do
New or worsening noises deserve a look the same day: check filter, vents and the outdoor unit for debris; tighten obviously loose panels (power off). Grinding, screeching, continuous banging or any noise with a burning smell: shut the system off and call. Buzzing and humming without startup: turn off and call, since running a stalled motor damages it. Hissing that persists: likely refrigerant; call. Note when the sound happens (startup, running, shutdown, defrost) and where; it shortens the diagnosis. If the noise comes with poor cooling, start with AC not cooling; winter heat pump noises are covered in heat pump blowing cold air. Repair prices are in the repair cost guide, and every contractor on the North Carolina registry shows the license status we found.