Short answer
Most noises point to a specific worn part: buzzing often means a failing capacitor or contactor, squealing a belt or motor bearing, banging a loose part or a blower problem. A dusty smell at the first heat of the season is normal and fades. A rotten egg smell means gas: leave the house and call the gas utility or 911 from outside. An electrical burning smell means shut the system off at the thermostat and breaker.
What's probably happening: noises
Buzzing or humming
A buzz from the outdoor unit, especially one that doesn't start, often means a failing capacitor or contactor. Those parts start the compressor and fan motor. Don't poke at it. A capacitor can hold a charge after the power is off. The good news is it's often a simple repair. From the Alpine Cool Facebook page: "A simple fix is the kind you want at 10pm. Fortunately for Cody it was just a capacitor!"
Banging or clanking
Something loose or broken: a blower wheel that has come loose, a failing motor mount, or a part hitting something it shouldn't. On a gas furnace, a loud bang at startup can be delayed ignition, which needs attention.
Squealing or screeching
Usually a motor bearing going bad, or a belt on older blowers. It tends to get worse, not better.
Rattling
Often a loose panel, a screw, or debris like sticks in the outdoor unit. Ductwork can also rattle as it expands and contracts.
Clicking
A click when the system starts and stops is normal. Repeated clicking with nothing starting can be a relay, a contactor, or a furnace trying to ignite and failing.
What's probably happening: smells
- Burning dust the first time the heat runs each season. Normal, and it should fade quickly.
- Electrical burning, hot plastic, or melting wire. Not normal. Shut it off.
- Rotten eggs or sulfur. Natural gas. Leave the house.
- Musty or mildew. Moisture somewhere: the drain, the pan, the coil, or ducts in a damp crawlspace.
What you can check yourself in five minutes
- Figure out where the sound is coming from: the outdoor unit, the indoor unit, or the ducts. Note when it happens, such as at startup, while running, or at shutdown.
- Look around the outdoor unit for sticks, leaves, or toys that could be hitting the fan. Turn the system off at the thermostat before clearing anything, and only reach what's outside the unit.
- Replace a dirty filter. A clogged filter can make the blower whistle and strain.
- Check that the furnace or air handler doors are on snug. A loose door rattles.
- For a musty smell, look for standing water in the drain pan and see whether the drain line is dripping outside.
- If you can, record a short video with sound. It helps a lot on the phone.
When to stop and call
- Rotten egg or sulfur smell: don't touch switches or the thermostat. Get everyone out, then call your gas utility or 911 from outside.
- Electrical burning smell or smoke: turn the system off at the thermostat and the breaker. If there's smoke or flame, get out and call 911.
- Loud buzzing with the unit not starting. Turn it off and leave it alone.
- Banging, grinding, or screeching that keeps going.
- Water on or near electrical parts. Shut the power off first.
- A breaker that trips again after one reset.
If you're in Spartanburg or Greenville County, turn it off, and call Alpine Cool at (864) 702-0777. Hold the phone up to the unit if it's still making the noise.
Common questions
Why does my heat smell like burning the first time I turn it on?
Dust settles on the heat strips or furnace parts over the summer, and it burns off the first time they heat up. A dusty burning smell that fades within a short while is normal. A smell that stays, or smells like hot plastic or wiring, is not.
My outdoor unit is buzzing but not running. Is that dangerous?
It usually means a part that starts the motors, like a capacitor or contactor, is failing. Turn the system off at the thermostat and leave the unit alone. A capacitor can hold a charge even with the power off.
What does a rotten egg smell from my furnace mean?
Natural gas has a rotten egg or sulfur smell added so leaks can be noticed. Don't touch switches or the thermostat. Get everyone out of the house and call your gas utility or 911 from outside.
Why does my house smell musty when the AC runs?
A musty smell usually comes from moisture: a slow drain, standing water in the pan, a damp coil, or ducts in a humid crawlspace. It's worth having the drain, pan, and ducts looked at.