Furnace making noise

Normal furnace sounds and noisy ones

A gas forced-air furnace has a few parts that make sound on every cycle. An inducer fan starts first to pull exhaust out, an igniter lights the burners, and once the heat exchanger warms up, the blower pushes air through the ducts. A click, a soft whoosh from the burners and a steady hum from the blower are normal. Ducts also tick and pop a little as the metal heats and cools.

New or louder sounds are the ones to track down. Air noises usually mean the system is starved for air, metal noises usually mean something has worked loose, and motor noises mean a belt or bearing is wearing out. Most of the easy causes are on the air side and outside the furnace cabinet. Noises from the burners or the motors themselves are where a technician comes in.

Common causes of furnace noise

Let's fix it

1. Pin down the sound and when it happensStand near the furnace through a full cycle. Notice whether the noise starts when the inducer fan starts, when the burners light, while the blower runs, or after it shuts off, and whether it comes from the furnace or out in the ducts. A bang or boom right as the burners light is delayed ignition; turn the furnace off at the thermostat and skip to the last step. Record a short video on your phone to remember the sound.
   Done

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2. Replace a dirty filterA clogged filter makes the blower strain, which can sound like whistling around the filter slot or a louder roar from the vents. Slide the filter out and hold it up to a light; if you can't see light through it, replace it with the same size, with the airflow arrow pointing toward the furnace. Make sure the filter cover is on so air isn't whistling in around the slot. We have a guide for changing an HVAC air filter for sizes and schedules.
   Done

   Something went wrong?

   • It still whistles with a new filter Check that the filter cover is on and the filter is the right size, then open the registers.

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3. Open vents and clear returnsClosing too many supply registers, or blocking return grilles with furniture or rugs, raises the pressure in the ducts. That can cause whistling at the registers and booming in the ducts when the blower starts. Open the registers in every room, clear anything in front of the return grilles, and listen again.
   Done

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4. Tighten loose panels and screwsRattles and buzzing usually come from loose sheet metal. Turn the furnace off at the thermostat, then check that the furnace's access panels are fully seated and their screws snug. Look for a loose screw, a loose duct strap or a rattling flue pipe connection, and tighten it. If a flue pipe joint has pulled apart or is rusted through, leave the furnace off and call a technician, since it can leak carbon monoxide. A strip of adhesive foam tape between two metal parts that touch can stop a buzz, but never block a vent opening or tape over the flue pipe.
   Done

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5. Check the ductsA pop or bang in the ducts when the blower starts or stops is often a flat section of duct flexing in and out, sometimes called oil canning. Follow the ducts you can reach in the basement or attic and look for loose joints, hanging straps that have come loose, and sections that flex when you push on them. Seal loose joints with foil HVAC tape, not cloth duct tape. If a large duct keeps booming, a technician can add a stiffening bend or brace.
   Done

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6. Look at the blower with the power offSquealing or scraping while the blower runs points to the blower. Switch off the furnace's power switch, usually on the side of the furnace or on a nearby wall, and its breaker, then confirm the power is off with a voltage tester. Remove the blower door. Look for debris inside the blower wheel and for a wheel that's loose on its shaft. On an older belt-drive blower, check the belt for cracks, glazing or looseness and replace it with one of the same size, set to the tension in the manual. A few older motors have oil ports; oil them only if the manual says to. Put the door back before restoring power.
   Done

   Something went wrong?

   • The blower door won't go back on Line up its bottom tabs first, then press it in until the door switch clicks; the furnace won't run without it.

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7. Call a technician for booms, grinding or screechingSome noises need a pro. A bang or boom when the burners light, grinding or screeching from the blower or inducer motor, a rumble after the burners shut off, or flames that lift, roll or look ragged all need service. Leave the furnace off until it's checked, and make sure your carbon monoxide alarms work; see the guide for a carbon monoxide alarm beeping. If you smell gas, leave the house and call the gas company from outside.
   Done

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Watch it done

Noisy Furnace? - Top 5 FixesBy AMRE Supply on YouTube. Their method may vary a little from ours.

Watch on YouTube

If it's more than noise

If the furnace runs but blows cold air, check our guide for a furnace blowing cold air.

If it turns on and off every few minutes, there's a separate guide for a furnace short cycling.

Furnace noise questions