The short answer
Some sounds are normal, like a heat pump's defrost whoosh or a click when the system starts. Grinding, screeching, loud banging, constant buzzing at the outdoor unit, a gas smell, or a sharp burning smell are not. For anything electrical or gas-related, turn the system off at the thermostat and breaker and call. For a gas smell, leave the house first.
Sounds and what they usually mean
| Sound | Usually | Urgency |
|---|---|---|
| Buzzing or humming at the outdoor unit, fan not spinning | Failed capacitor or contactor, seized fan motor | Turn off, call soon |
| Squealing or screeching | Blower or fan motor bearings, a worn belt on older furnaces | Call soon |
| Grinding or scraping | Motor bearings failing, or a fan blade hitting something | Turn off, call |
| Loud bang when the furnace lights | Delayed ignition from dirty burners | Turn off, call |
| Rattling | Loose panel, debris in the outdoor unit, or ductwork expanding | Check panels, call if it persists |
| Hissing near the lines or coil | Possible refrigerant leak or a duct leak | Call |
| Whoosh and steam from a heat pump in winter | Normal defrost cycle | None |
Smells and what they usually mean
- Rotten eggs or sulfur: natural gas. Leave the house and call the gas utility or 911 from outside.
- Sharp electrical, hot plastic: an overheating motor or wiring. Turn the system off at the breaker.
- Dusty burning, first cold day: normal dust burn-off. It should fade within an hour.
- Musty or mildew: moisture in the coil, pan, or ducts. Common in our humidity.
- Exhaust smell with a gas furnace: possible venting problem or cracked heat exchanger. Turn it off and check your CO alarm.
When to call
If a sound is new and getting louder, or any smell is electrical, gas, or exhaust, call Thermal Services at (704) 701-6478. Catching a failing motor early is usually far cheaper than replacing what it takes down with it.
Common questions
Why does my heat smell like burning the first time I turn it on?
Dust settles on the heat exchanger or heat strips over the summer and burns off the first few times the heat runs. A faint dusty smell that fades within an hour is normal. A strong, sharp electrical or plastic smell is not; turn the system off and call.
What does a musty smell from the vents mean?
Usually moisture: a dirty indoor coil, a wet drain pan, or mold growth in ducts or insulation. In our humid climate this is common. Change the filter, check the drain pan, and have the coil and pan cleaned if the smell persists.
Why does my AC make a loud click and then nothing happens?
A click followed by a hum or silence at the outdoor unit often means the contactor pulled in but the compressor or fan motor didn't start, frequently a failed capacitor. Turn it off at the thermostat and have it checked before the compressor overheats.
My outdoor unit sounds like a whoosh and steam in winter. Is that normal?
Yes, that's a heat pump defrost cycle. The outdoor fan stops, the reversing valve shifts with a whoosh, and steam rises off the coil for a few minutes.