Short answer
Ticks and pops from the ducts as they warm up are normal. A whistle usually means a clogged filter or closed vents, and a rattle can be something touching the furnace. A loud boom when it lights, a squeal, grinding or scraping are not normal: turn the furnace off at its switch and call a technician.
Safety first
A loud bang or boom when the furnace starts can mean gas builds up before it lights. Turn the furnace off and call. If you smell gas, do not touch switches: get out and call 911 or your gas company from outside.
How to tell which furnace noises need a pro
1Bangs, squeals or rattles from the furnace
Every furnace makes some noise. The trick is telling a normal tick from a sound that needs a technician.
Read all the steps
- Bangs, squeals or rattles from the furnace. Every furnace makes some noise. The trick is telling a normal tick from a sound that needs a technician.
- Whistling? Replace the filter. A clogged filter makes the blower pull hard and whistle. Put in a new one of the same size and open any closed vents.
- Rattling? Clear things away from it. A broom, a box or a shelf touching the furnace rattles with the blower. Move them away and keep three feet clear.
- Ticks and pops from the ducts are normal. Metal ducts tick and pop as they warm up and cool down. You hear it most when a cycle starts and ends.
- Boom, squeal or grinding? Off, then call. A boom when it lights, a squeal, grinding or scraping metal mean a part is failing. Turn the furnace switch off and call a technician.
- Still not fixed? Call an HVAC contractor Especially if: it makes a boom or a bang when it lights.
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What the noise usually means
- Boom or bang when it starts: delayed ignition, the gas lights late. Turn it off and call.
- Squealing or screeching: the blower motor or its bearings, or a worn belt on older furnaces.
- Grinding or scraping metal: the blower wheel or the motor bearings. Turn it off.
- Rattling from inside the cabinet: a loose part or the inducer motor.
- Humming or buzzing: a motor, a capacitor or the transformer.
- Clicking over and over without lighting: the ignition is failing.
What not to do
- Do not open the panels to look for the noise while the furnace has power.
- Do not oil motors or tighten parts inside the cabinet.
- Do not ignore a boom at startup. Over time it can crack the heat exchanger, which can leak carbon monoxide.
When to call an HVAC technician
- It makes a boom or a bang when it lights.
- You hear squealing, grinding, scraping or a loud hum.
- The rattle comes from inside the cabinet.
- A new noise comes with a burning smell or soot around the furnace.
Frequently asked questions
Why does my furnace bang when it turns on?
A loud bang right when it lights is often delayed ignition: gas builds up for a moment before it catches. Turn the furnace off and call a technician. Softer pops from the ducts as they warm up are normal.
Is it normal for a furnace to make noise?
Some noise is normal: a click when it starts, the hum of the blower, ticks from the ducts. Booms, squeals, grinding and loud rattles are not.
Why is my furnace squealing?
Usually the blower motor or its bearings, or a worn belt on older furnaces. Turn it off and call a technician before the motor fails.
Why does my furnace rattle?
Often something touching the cabinet or a loose outer panel. If the rattle comes from inside, have a technician check the motors and the heat exchanger.
Can a dirty filter make my furnace noisy?
Yes. A clogged filter makes the blower strain and can cause a whistle. Replace it and the noise often goes away.