Why Does My Furnace Rattle or Rumble When It Starts?

Quick answer: A brief rattle as the blower spins up is often a loose panel, screw or duct connection. A rumble or boom as the burners light can mean delayed ignition, where gas builds up before it lights, and that needs a technician soon. If it’s loud or you smell gas, shut the furnace off and call.

Why is my furnace loud when it starts?

Usually something loose is rattling as air starts to move, or the burners are lighting late. Timing is the best clue, because a gas furnace starts in steps. First, a small inducer motor, a fan, hums to clear the burner area of old air.

Next, the igniter (the part that lights the gas) glows or sparks, and the burners light with a soft whoosh. Within about a minute, the main blower starts pushing warm air into the ducts. In most Austin homes this happens in the attic, so you may hear it only through the ceiling.

Why does my furnace rattle when the blower starts?

Usually something loose or dirty:

• A loose panel or screw. An access panel that isn’t fully seated, or loose cabinet screws, can chatter.
• A loose plenum joint. In Central Texas the plenum (the box atop the furnace that feeds the ducts) is usually fiberglass ductboard, sometimes sheet metal, and a loose joint or sheet metal transition can buzz as air pressure hits it.
• A dirty blower wheel. The blower is the big fan that pushes air to the rooms. Caked-on dust can throw it off balance and cause a rumble or vibration.
• Worn motor parts. Tired motor mounts, or an inducer motor with worn bearings, can rattle as they spin up.
• A loose filter door or grille. Either one can vibrate in the airflow.

Reseating the panel may quiet it. Anything inside the cabinet is for a technician, since it houses gas, flame and full household-voltage wiring.

Why does my furnace boom when it turns on?

Often, delayed ignition. The burners don’t light right away, a little extra gas collects, and then it all lights at once with a thump. Dirty or rusty burners (especially the crossover that carries flame from burner to burner), a weak igniter, a partly blocked vent and gas pressure problems are the usual causes. A furnace that sat all summer in a dusty attic is a prime candidate.

Repeated delayed ignition is hard on the heat exchanger, the metal chamber that keeps combustion gases out of your air. A rumble that continues while the burners run can also point to a burner or flame problem.

When should you shut the furnace off right away?

Turn it off at the thermostat and call:

• When you hear a loud bang or boom as the burners light.
• When you smell gas. Leave the house and call from outside, as covered in what to do if you smell rotten eggs near your furnace.
• When a carbon monoxide alarm sounds. See why a CO alarm beeps or chirps.
• When you see scorch marks, or flames outside the burner area.

Is a furnace boom the same as ducts popping?

No. Duct popping comes after warm air starts flowing, when metal parts expand and flex, and why ducts pop when the heat comes on covers it. Ductboard plenums and flex runs pop less, but sheet metal plenums and fittings can still tick. A boom at the moment of ignition is a burner issue, not a duct issue.

Can a tune-up prevent a furnace boom?

It helps. ENERGY STAR’s maintenance checklist for gas furnaces includes checking gas connections, gas pressure, burner combustion and the heat exchanger, which is where delayed ignition gets caught. What’s included in an HVAC tune-up walks through the rest. Gold Eagle handles heating tune-ups and burner repairs around Austin.

What should you do next?

• Note when the noise happens: at ignition, when the blower starts or after warm air flows.
• Check that the furnace access panel is fully seated.
• Shut the furnace off and call if you hear a boom or smell gas.
• Book a heating tune-up before the first cold snap.

Related guides: What’s Included in an HVAC Tune-Up?, Why Does My Furnace Turn On and Then Shut Off After a Few Seconds? and Heater Smells Like Burning? What’s Normal and When to Call.

About Gold Eagle Services: This guide comes from the team at Gold Eagle Services, a family-run Austin HVAC, indoor air quality and mold remediation company founded in 2017. We’re licensed (TACLA119235 and RCO1783), insured and bonded, and we serve homes across the greater Austin area.

Need help with this in your home? Learn about our heater repair and tune-ups, or call 512-489-4653 to schedule a visit.