The short answer
A boom or bang when the burner lights can mean delayed ignition: gas builds up before it lights. Stop using the furnace and call a technician; if you smell gas, leave and call from outside.
Scraping or grinding metal means turn it off now. Squealing, rattling and humming usually point to motors, loose panels or electrical parts; whistling often means a clogged filter or blocked airflow.
Safety first: booming at startup
A loud boom, bang or small “explosion” sound just as the burners light is often delayed ignition. Gas flows for a moment before it lights – because of dirty burners, a weak igniter or a gas pressure problem – and lights all at once. Repeated hard starts can damage the heat exchanger, which keeps exhaust out of your air.
- Turn the thermostat down so the furnace stops calling for heat and call a technician.
- If you smell gas, leave the house first and call 911 or your gas utility from outside.
- Make sure your carbon monoxide alarms work – see the carbon monoxide safety guide.
Noise guide
| Noise | Usual cause | What to do |
|---|---|---|
| Boom or bang when the burner lights | Delayed ignition: dirty burners, weak igniter or gas pressure | Stop using it and call. Leave if you smell gas. |
| Squealing or screeching | Worn blower motor or inducer bearings, or a slipping belt on older furnaces | Call. Running a dry bearing can seize the motor. |
| Scraping or grinding metal | Blower wheel loose or rubbing its housing, or a failing motor bearing | Turn the furnace off now and call. |
| Rattling or vibration | Loose access panel, screws, ductwork or a part inside starting to fail | Push the front panel firmly into place. If the rattle comes from inside, call. |
| Clicking at startup | Normal relays and the igniter. Repeated clicking with no flame is an ignition problem | Note the blink code and call if it does not light. |
| Humming or loud buzzing | Transformer, motor or capacitor; a loud hum with no motor running suggests a failing part | Call. Capacitors store a dangerous charge. |
| Whistling | Clogged filter, closed registers, blocked returns or a duct leak | Replace the filter and open vents. Call if it continues. |
| Popping or ticking in ducts | Metal ducts expanding and shrinking as they heat and cool | Usually harmless. Ask about it at the next tune-up if it is loud. |
| Gurgling | Water in the condensate drain or trap of a high-efficiency furnace | Check for water around the base; a technician can clear the trap. |
Sounds that are normal
A typical heating cycle has a few sounds of its own: a click as the thermostat calls for heat, the hum of the inducer fan clearing the vent, a soft whoosh as the burners light, then the steady rush of the blower. When the burners go out, the blower keeps running briefly to use up remaining heat. Ducts may tick as they warm and cool. What matters is a change: a sound that is new, louder or harsher than it used to be.
Squealing, screeching and scraping
Most modern furnaces drive the blower directly from the motor, so a squeal usually points to worn motor bearings. Older furnaces may have a belt that slips or frays. A metal-on-metal scrape or grind often means the blower wheel has come loose or a bearing is failing, and it can get worse quickly. Turn the furnace off at its switch and call. Do not oil or adjust parts inside the cabinet yourself.
Rattles and vibration
Start with the simple fix: the front access panel. If it is not seated, it can rattle – and on many furnaces a door switch keeps the furnace off until the panel is closed. Check screws on the cabinet and the metal ducts you can see. A rattle from inside the cabinet, or one that started suddenly and loudly, can be a loose motor mount, blower wheel or inducer and needs a technician.
Clicking and humming
One or two clicks at the start and end of a cycle are relays and the gas valve doing their job. A furnace that clicks over and over without lighting is trying and failing to ignite. Count the blinks on the control board light and tell the technician. A steady hum is normal from motors; a loud buzz, or a hum when nothing seems to be running, can be a failing capacitor, motor or transformer. Leave electrical parts to a professional.
Whistling and airflow
Whistling usually means air is being forced through a gap that is too small. A clogged filter is the most common cause; ENERGY STAR advises inspecting, cleaning or changing filters once a month. Also open closed registers, clear blocked returns and listen at the filter slot, which may need its cover. Whistling from a duct joint can point to a leak – see duct sealing.
When to call
- Booming or banging when the burners light.
- Scraping, grinding or screeching.
- Clicking with no flame, or an error code.
- Loud humming or buzzing, or a burning smell with the noise – see furnace burning smell.
- Any noise along with a gas smell or a CO alarm: leave first, then call from outside.
A yearly inspection catches many of these early. The U.S. Consumer Product Safety Commission advises annual professional inspection and service of the heating system, and ENERGY STAR’s contractor checklist includes burner combustion and the heat exchanger. See furnace maintenance.
Sources
- ENERGY STAR: HVAC Maintenance Checklist (checked October 2026)
- U.S. Consumer Product Safety Commission: Carbon Monoxide Questions and Answers (checked October 2026)
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Questions
Why does my furnace make a loud bang when it turns on?
Often delayed ignition: gas collects for a moment before lighting, then lights all at once. It can strain the heat exchanger. Stop using the furnace and call a technician; if you smell gas, leave and call from outside.
Why is my furnace squealing?
Usually worn blower or inducer motor bearings, or a slipping belt on an older furnace. Call a technician before the motor seizes.
Is it normal for a furnace to click?
A click or two at the start and end of a cycle is normal. Repeated clicking without the burner lighting means an ignition problem.
Why is my furnace whistling?
Most often a clogged filter or blocked airflow – closed registers, blocked returns or a missing filter cover. Replace the filter and open vents first.
Related guides
Updated October 2026 by the Heating and Cooling editorial team. General information for homeowners, not professional advice – rules and equipment differ. When gas, carbon monoxide, electricity or anyone’s safety is involved, call 911 or your utility. How we write our guides · HVAC glossary