If you only do one thing: if your furnace makes a bang or a boom when it lights, turn it off at the thermostat and call. That is the one sound on this page that should not wait for a convenient day.
Almost every other furnace noise is a part telling you it is wearing out, and most of them give you weeks of warning. Here is what each one usually is.
The sound, and what it usually means
| The sound | What it usually is | Normal? | If it needs us |
|---|---|---|---|
| A click, a whoosh, then the fan a minute later | A healthy start: relay, burners lighting, blower delay | Yes | Nothing |
| Ticks and pops in the walls as it heats and cools | Metal ducts expanding and shrinking | Yes | Nothing, unless it is one loud bang |
| A bang or boom when it lights | Delayed ignition: gas building up before it catches | No. Turn it off | Diagnostic $99, then the repair in writing |
| Clicking over and over, no flame | Igniter or the flame not being sensed | No | Igniter $240 to $410, sensor cleaned $95 to $180 |
| A high squeal or screech | Blower motor or inducer motor bearings | No | Blower motor $670 to $2,400; inducer quoted after we look |
| Grinding or scraping metal | A motor failing, or the blower wheel loose and rubbing | No. Turn it off | Priced at the visit |
| A loud hum, and nothing starts | A capacitor, or a motor that will not turn | No | Capacitor $240 to $370 |
| Rattling | A loose panel, screw or duct | Usually harmless | Often a few minutes on a visit |
| Whistling | Air squeezing through: dirty filter, closed vents, a duct gap | No, but often yours to fix | Nothing, if a filter fixes it |
| Thumping or flapping | The blower wheel out of balance, or something caught in it | No | Blower wheel cleaned $300 to $440 |
| Gurgling on a high efficiency furnace | Water in the condensate trap or drain | A little, yes | Drain or trap $175 to $420 if it shuts off |
Our Northern Virginia prices, the same price book as the full furnace repair list.
Normal sounds first, because they cause the most calls
A gas furnace starting up makes a little sequence of noises, and once you know it you will stop hearing it as trouble. A click as the thermostat calls. The inducer fan spinning up, a small motor that clears the flue. The igniter glows, the burners light with a soft whoosh, and about a minute later the main blower comes on. When the thermostat is satisfied, the burners go out and the blower keeps running for a minute or two to pull the last heat off.
The ticking and popping you hear inside the walls is usually the ductwork. Sheet metal grows a little when warm air hits it and shrinks again when it cools, and a long straight run can pop like a cookie sheet in an oven. Annoying, harmless. What is not harmless is one loud boom right at the furnace, which is different.
The bang: delayed ignition
A furnace that lights with a bang has gas flowing for a moment before it catches, so a small amount builds up and lights all at once. The usual causes are dirty burners, an igniter that is getting weak, or gas pressure that is off. Every one of those gets worse, not better, and a small ignition bang every cycle is hard on the furnace.
Turn the heat off at the thermostat and call. If you also smell gas, leave the house first and call 911 and the gas company from outside.
Squeals, screeches and grinding
A furnace has two motors. The big one, the blower, moves air through the house. The little one, the inducer, pushes exhaust up the flue before the burners are allowed to light. When either one’s bearings start to go, you get a squeal, a whine, or a screech that is worst at startup and fades as the motor warms up. That fade is the motor buying you time, not healing.
Older furnaces with a belt-driven blower squeal when the belt glazes or slips. Most furnaces built in the last few decades are direct drive and have no belt, so a squeal from one of those is usually a bearing.
Grinding or metal scraping is the next stage. Either the bearings are gone or the blower wheel has worked loose on its shaft and is rubbing the housing. Turn it off. A motor that seizes completely can take the control board or the capacitor with it, and a blower wheel that comes apart turns a motor job into a bigger one. A blower motor runs $670 to $2,400, and the spread is a standard motor against a variable speed one. The inducer is quoted after we look, because it depends on the furnace.
Humming and clicking
A low hum from the furnace is normal. The transformer that runs the controls hums, and so do motors. A loud hum with nothing happening after it is different: the motor is getting power and not turning, which is usually a failed capacitor, the small part that gives a motor its push to start. It is one of the cheaper repairs on the list, $240 to $370, and it is worth doing before the motor overheats trying.
One click at startup is the relay. Clicking over and over with no flame is the furnace trying to light and failing: a cracked igniter, or burners that light and go out because a dirty flame sensor cannot see the flame. Most furnaces try a few times and then lock themselves out for a while. Cleaning the sensor runs $95 to $180. A new igniter, $240 to $410. How the flame sensor works, and why it fails. If the clicking comes with cold air, this is the page for that.
Rattling, whistling and flapping
Rattling is usually the least serious noise a furnace makes. A front panel not fully seated, a screw backed out, a duct strap loose, or the sheet metal of the cabinet buzzing against something. Push gently on the panels while it runs and see if the rattle changes. If it is coming from the outdoor unit on a heat pump, it is often a loose fan guard or a panel there.
Whistling is air being squeezed. Start with the filter: a clogged one makes the blower pull hard through whatever gaps it can find. Then walk the house and open any closed registers, because closing them does not save money on most systems and makes the rest of the ducts work harder. A whistle at one spot in the ductwork is usually a gap where two pieces meet. Which filter to buy, and why a thicker one is not always better.
Thumping or flapping in time with the blower is a wheel out of balance. Usually that is years of dust packed unevenly on the blades. Sometimes it is a filter that got sucked into the blower compartment. Turn it off and look before running it again. A blower wheel pulled and cleaned runs $300 to $440, and through through December 31 our blower wheel deep clean does it with a full tune-up for $229.
Gurgling, and the heat pump sounds people mistake for the furnace
A high efficiency furnace, the kind with white plastic pipes, makes water every time it runs, and you will hear it moving through the trap and drain. A little gurgle is normal. A loud one together with the furnace shutting off is a drain that is backing up, and that drain is worth reading about before January.
If you have a heat pump, a loud whoosh followed by quiet, and steam rising off the outdoor unit, is the defrost cycle. It is the system reversing for a few minutes to melt frost off the outdoor coil. Normal, and it will do it many times on a cold damp day. A buzz from the outdoor unit with the fan not turning is not normal: that is usually a contactor ($290 to $425) or a capacitor. How a heat pump behaves in a Virginia winter.
When a noise means turn it off now
- A bang or boom when it lights.
- Grinding or scraping metal.
- Any noise together with a burning electrical smell. Kill the breaker.
- Any noise together with a gas smell. Leave, then call 911 and the gas company from outside.
- Your carbon monoxide alarm. Get everyone out and call 911. What to do next.
Everything else can usually wait for a weekday appointment, and a furnace past fifteen with a failing motor is worth pricing against replacement before you pay for the motor. Our furnace estimator gives you that number for your house size, $4,285 to $14,040 installed. From about $71 a month at 0% APR for 60 months, the whole price financed, on approved credit. What size furnace your house needs.
Book a repair, or see what a new one costs
Need it fixed? The diagnostic visit is $99, and you get the repair price in writing before any work starts. Book a visit online or call (703) 997-9757, day or night.
What a replacement costs in your house. Answer a few questions and see the installed price for your size: the furnace estimator for a gas furnace (from $4,285) and the heating and cooling estimator for a heat pump or a furnace and AC together (from $8,499). From about $71 a month at 0% APR for 60 months, the whole price financed, on approved credit. Each one lets you book a free in-home measurement.
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