Furnace Making Noise: What Each Sound Means

Every furnace makes some noise. A soft click, the burners lighting with a quiet whoosh, the blower spinning up. What you want to listen for is a new sound, or a sound that's getting louder. The type of noise usually points to the part that's wearing out. This guide walks through the common sounds, which ones you can check yourself and which ones mean you should shut the furnace off. LocalRepairLine is a referral service, not a contractor, and this is general information, not a substitute for an in-person inspection.

What the sound usually means

• A boom or bang when the heat comes on. Often delayed ignition. Gas builds up for a moment before it lights, which usually points to dirty burners or an ignition problem. Don't keep running it.
• Popping or ticking in the ducts. Metal ductwork expands as it heats and shrinks as it cools. Usually harmless, though very loud pops can mean a duct that needs support.
• Squealing or screeching. Commonly a worn blower belt on older furnaces or a blower motor bearing that's going dry.
• Grinding or scraping. Metal on metal, often the blower wheel rubbing or a motor bearing that has failed. Shut it off.
• Rattling. A loose access panel, a loose screw in the cabinet or ductwork that has come loose.
• Humming or buzzing. Can be normal transformer hum, but a loud buzz can mean a failing motor, a bad capacitor or an electrical issue.
• Repeated clicking with no heat. The ignitor or the controls are trying to light the furnace and failing.
• Whistling. Usually an airflow problem, like a clogged filter or closed vents.

Safe checks to run first

1. Notice when it happens. At startup, while it runs, or when it shuts off? That one detail narrows it down a lot.
2. Check the filter. A filter packed with dust makes the system strain and whistle. Replace it if it's dirty.
3. Open the vents. Make sure supply vents and return grilles aren't closed or blocked by furniture.
4. Make sure the panel is on tight. A loose front panel rattles, and on many furnaces it also has to be seated for the furnace to run.
5. Test your CO alarms. Press the test button on each one, especially near bedrooms and the furnace.

What not to do

• Do not keep running a furnace that booms, grinds or smells like burning.
• Do not take apart the burners, gas valve or blower yourself.
• Do not oil a motor unless you know it has oil ports and the power is off.
• Do not ignore a noise that keeps getting louder. Small problems often turn into a no-heat call on the coldest night.

When to call a furnace tech

• You hear a boom at startup, grinding or loud screeching.
• The noise comes with a burning smell, a tripped breaker or no heat. Our furnace not heating checklist covers the no-heat side.
• The furnace clicks over and over but never lights.
• A CO alarm has gone off, even once.
• The sound is new and the filter, vents and panel all check out.

What to tell the tech

• What the sound is like: bang, squeal, rattle, hum, click.
• When it happens and how long it's been going on.
• Whether the house is still heating normally.
• What you already checked, and roughly how old the furnace is.

Loud banging furnace guides by city

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