A furnace that has run quietly for years suddenly announces itself — a deep boom a few seconds after it lights, a metallic rattle that shakes the hallway closet, a squeal that rises with the blower. Noise is the one symptom a furnace gives you before it fails, and the sound itself tells a tech most of what they need to know. Some of these noises are harmless. One — a bang on ignition — is a genuine safety problem and a reason to shut the furnace off.
How it usually shows up:
- A loud bang or boom a few seconds after the furnace starts, sometimes strong enough to rattle the ductwork.
- A rattle or buzz from the cabinet whenever the blower runs.
- A squeal or screech that starts with the blower, or a grinding that worsens week to week.
- Pops and ticks in the walls and ceilings shortly after the heat kicks on.
What each sound usually means
Each noise points to a different part of the furnace:
- BANG or BOOM on startup — delayed ignition. The important one. Gas enters the burners, doesn't light immediately, and pools until it ignites all at once. Causes are dirty or rusted burners, misaligned ports, a weak hot-surface igniter, or gas pressure off spec. Repeated booming stresses the heat exchanger and can crack it — which is how combustion gases including carbon monoxide reach the air you breathe.
- RATTLING — loose hardware or a failing blower wheel. Often a panel that isn't seated, loose sheet-metal screws, or a duct vibrating against a joist. A rattle that tracks with blower speed points instead to a wheel out of balance, caked with dust, or loosening on its shaft.
- SQUEAL or SCREECH — blower bearings or a belt. On older belt-drive furnaces a slipping or cracked belt squeals as it loads up; on direct-drive blowers it means bearings going dry. A warning, not an emergency — but the motor is on the clock.
- GRINDING — motor bearings, and stop running it. Metal on metal. Once bearings are this far gone, running the furnace is what destroys the motor and sometimes the blower housing with it.
- RUMBLING while idle — a burner or pilot problem. A rumble after the burners should be off, or a rolling flame sound mid-cycle, points to dirty burners, a mis-set pilot, or incomplete combustion.
- POPPING in the ducts — usually normal. Sheet metal expands as hot air enters and contracts as it cools. Not a furnace fault, though a clogged filter makes it worse by raising static pressure.
The usual fix
Our tech confirms the noise, then follows the furnace's sequence of operations to place it. For a bang we watch a real ignition cycle: how long the igniter takes to glow, whether the burners hesitate, and whether the flame carries over evenly. Delayed ignition is usually resolved by cleaning or realigning the burners and replacing a weak hot-surface igniter, with gas pressure verified at the valve. Mechanical noises are simpler — reseat panels, clean or rebalance the blower wheel, or replace a motor that's squealing or grinding. After repeated booming we also inspect the heat exchanger for cracking, which is why a boom gets a same-week appointment rather than a wait. You get a firm, flat-rate quote before any part goes in.
DIY vs. calling a pro — the honest version
Safe to check yourself first (free):
- Change the air filter. A restricted filter raises duct pressure, which makes popping, whistling, and blower noise dramatically worse.
- Check that the front panel is seated and the cabinet screws are snug. An unlatched panel buzzes against the housing whenever the blower runs.
- Clear the return — nothing leaning on the return grille, no boxes stacked against the cabinet.
- Note when the noise happens: at ignition, only with the blower, or all the time. That one detail shortens the diagnosis considerably.
Call a pro when:
- You hear any bang or boom on ignition, even occasionally — that's the heat-exchanger and carbon-monoxide risk.
- The furnace is grinding, or squealing loudly enough to hear from another room.
- There's a burning smell with the noise, or you smell gas at any point.
- The noise came with weaker heat, short cycling, or the furnace shutting itself down.
If the furnace booms repeatedly, stop using it until it's inspected and make sure you have working carbon monoxide alarms. If you smell gas strongly, leave first and call your gas utility or 911 — don't investigate.
What it costs in the Bay Area
The cost follows the part, not the volume of the noise. In our pricing, a hot-surface igniter runs about $120–$350 and a flame sensor $90–$250 — the parts most often replaced when delayed ignition is the cause, usually alongside a burner cleaning. A blower motor is roughly $400–$700, covering the squealing and grinding cases. Tightening panels and clearing an airflow restriction sit at the low end. We charge a diagnostic fee that's credited toward the repair when you approve the work, always with a firm, upfront price first.
Get the noise diagnosed
Triton repairs gas and electric furnaces across the Peninsula and East Bay as part of our full HVAC repair service — San Mateo, Burlingame, Redwood City, Alameda, San Leandro, and Hayward — where older homes and closet or hallway installs put the furnace close enough to living space that every new sound gets noticed. Same-week and often same-day appointments, upfront pricing, licensed techs.
Book online or call (888) 993-5699. Tell us what the noise sounds like and when it happens, and we'll bring the right parts.
Related pages
- HVAC Repair in the SF Bay Area — our full heating & cooling hub
- Furnace Not Heating / No Heat — when the noise comes with no warm air
- Furnace Won't Ignite — the delayed-ignition and no-light case
- Furnace Short Cycling (On and Off) — lights then quits, over and over
- HVAC & Furnace Repair in San Mateo — Peninsula service area
Frequently asked questions
Why does my furnace make a loud bang when it starts? That's delayed ignition. Gas enters the burners but doesn't light right away, pools for a second or two, and then ignites all at once — the bang is that small explosion. It's usually dirty or rusted burners, a weak hot-surface igniter, or gas pressure that's off. Repeated booming can crack the heat exchanger, so it should be inspected rather than ignored.
Is a banging furnace dangerous? It can be. The bang is a pressure event inside the furnace, and repeating it stresses the heat exchanger. A cracked heat exchanger lets combustion gases, including carbon monoxide, mix with the air moving through your house. If your furnace booms on startup, stop using it, make sure you have working CO alarms, and have it looked at.
Why is my furnace rattling? Usually something loose — a front panel that isn't seated, loose cabinet screws, or a duct vibrating against framing. If the rattle changes with blower speed, it's more likely the blower wheel: out of balance, packed with dust, or loosening on its shaft. Check the panel and change the filter first.
Why does my furnace squeal or screech when it starts? A squeal that comes and goes with the blower points to the blower motor — a worn belt on older belt-drive units, or bearings drying out on direct-drive blowers. It's an early warning rather than an emergency, but a squeal that turns into grinding means the motor is failing and should be shut off.
My ducts pop when the heat comes on. Is that the furnace? Usually not. Sheet-metal ducts expand when hot air hits them and contract when they cool, and that flexing pops and ticks. A dirty filter or restricted ductwork makes it louder by raising pressure in the system. If the popping is joined by banging at the furnace itself, that's a different problem.
How much does it cost to fix a noisy furnace in the Bay Area? It depends on the part. A hot-surface igniter is about $120–$350 and a flame sensor $90–$250 — the usual delayed-ignition repairs — while a blower motor for squealing or grinding runs roughly $400–$700. Tightening loose hardware or clearing an airflow restriction is at the low end. We credit the diagnostic fee toward the repair and quote a firm price before any work begins.