Top-down view of an outdoor condenser fan grille

AC noises & smells · Pueblo, CO

Strange AC noises and smells

Your air conditioner usually tells you before it quits. Here’s what each sound and smell means, which ones mean shut it off, and when to call us.

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Shut it off now if…

Burning smell, smoke or sparks at the unit? Cut the power first.

Turn the system off at the thermostat, then switch off the air conditioner’s breaker in your electrical panel. Don’t open the outdoor unit: its capacitor can hold a dangerous charge even with the power off. If you see flames or smoke that keeps coming, get everyone out and call 911. Once the power is off and everyone is safe, call us.

Rotten-egg smell? That’s gas, not the AC.

Natural gas has an added odor like rotten eggs or sulfur. If you smell it near the furnace or water heater, leave the house now. Don’t flip switches, use a garage door opener or make calls inside. From outside, call 911 or Xcel Energy’s 24/7 gas emergency line, 1-800-895-2999. In the part of Pueblo West served by Colorado Natural Gas, call 911 and use the emergency number printed on your gas bill. Don’t call us first — the gas utility needs to make it safe.

Also turn the system off for banging, clanking or screeching from the outdoor unit, or hissing near the refrigerant lines. Those can turn a repair into a replacement if the system keeps running.

Sounds

Sound	What it usually means
Loud hum or buzz, fan not spinning	A failing capacitor or seized fan motor. The unit is trying to start and can’t. Turn it off.
Clicking at the outdoor unit	Often the contactor or capacitor; hard starting with loud clicks can point to compressor electrical trouble.
Clicking at the indoor unit	A control board, loose connection or safety switch, like a float switch cutting power.
Hissing	Never normal. Near the indoor coil or the copper lines, it can mean a large refrigerant leak. Shut the system off.
Squealing or grinding	Worn fan motor bearings, or a slipping belt on older equipment.
Banging or clanking	Something loose or broken inside the unit. Lennox lists banging and clanking among the sounds that call for shutting the system down right away.
Rattling	Loose panels or screws, or debris in the outdoor unit. Usually simple, but don’t reach inside a running unit.

Typical U.S. range for a fan motor repair: $100–$700, per Angi, October 2026 — a national figure, not a Pueblo quote. Your exact price comes after diagnosis.

Smells

Musty or “dirty sock”

Moisture is sitting where it shouldn’t — on the indoor coil, in the drain pan, or in the ducts — and mold or mildew is growing. A clogged drain line and a dirty filter are common reasons the moisture stays put. Cleaning the coil and pan, clearing the drain and changing the filter usually fixes it; if it doesn’t, we look at the ducts.

Burning or hot electrical smell

An overheating motor or wiring. That’s a fire risk: turn the system off and switch off its breaker before you call.

Burning dust when the heat first comes on

If the smell shows up when the heat runs for the first time in fall, it’s usually dust burning off and should fade quickly. That’s a heating question; if it lingers, it’s worth a call either way.

Smoke

Treat any smoke smell from the AC as electrical until proven otherwise: power off at the breaker, then call.

Questions we hear

Is some noise from the outdoor unit normal?

Yes. A steady hum while it runs is normal, and the outdoor unit makes a bit more noise when it starts up or changes speed. What isn’t normal: a hum with no fan movement, hissing, banging, grinding, screeching, or rapid clicking that never turns into a start.

I smell rotten eggs near the furnace while the AC is running. Is that the AC?

No — treat it as a possible natural gas leak. Leave the house without flipping switches or using your phone inside, then call 911 or your gas utility’s emergency line from outside (Xcel Energy: 1-800-895-2999). An air conditioner does not make that smell. Once the gas utility clears the house, we can look at the cooling system.

Can I tell a refrigerant leak by the smell?

Not reliably. Bryant cautions that a chemical odor has several possible sources and shouldn’t be assumed to be refrigerant from smell alone. Hissing near the lines, ice on the coil and fading cooling are better clues. A technician confirms a leak with instruments.