Noisy Ducts: Banging, Popping and a Loud Vent

By Ray Bennett · Updated 2 October 2026 · 6 min read · General guidance, always check your own manual

Noisy Ducts: Banging, Popping and a Loud Vent

Short answer

A boom a second after the blower starts is a flat panel of duct bowing under the pressure and snapping back, and it happens again when it stops. Ticking a few minutes in is metal growing as it warms. A whistle or a roar is too much air through too little duct. None of it is the machine, and some of it is free to cure.

Duct noise gets blamed on the furnace or the air conditioner, and then somebody looks at the furnace and finds nothing wrong with it. The noise is coming from a few hundred pounds of thin sheet metal that changes shape every time the fan starts.

The good news is that ducts are honest about it. Where the noise happens and when it happens point straight at which of four things it is.

Before you start

What to carry: a torch, a screwdriver and a new filter. Twenty minutes, plus one full cycle to listen to.

Gas or electric? The ducts do not care. A gas furnace, an electric furnace and an air handler all push air through the same metal, so every noise here belongs to all three. The one difference is that a gas furnace makes combustion noises too, and those are a separate list.

Turn the thermostat up or down five degrees so you get a proper start, a run and a stop rather than catching the end of one.

One noise that is not the ducts, and one thing not to do. A bang at the moment the burners light. That is delayed ignition inside a gas furnace, not ductwork. It is a small explosion every time it starts, it cracks heat exchangers, and it does not belong on this page at all: what that bang actually is.

Never close or block a return to quieten it. The return is how the system breathes in, and starving it makes every other noise on this page worse. On an older furnace that vents up a chimney it can also pull exhaust back into the house.

Where along the run

Four places, four noises. Stand at each in turn rather than guessing from the sofa.

Check 1

Walk the run once, and note the timing

Stand at the furnace or air handler first, then follow the big duct as far as you can see it, then stand at a register.

At the moment the blower starts, and again when it stops. A flat panel of duct flexing. That is check 2.

A few minutes in, then again a few minutes after it stops. Metal expanding and contracting, ticking where it rubs on something.

All the time it runs, steady. Air, not metal. That is either a whistle at one place or a roar everywhere, and the last two sections cover them.

Only at one register. The register itself, which is the easiest fix on the page.

A hum or buzz that matches the blower exactly. Vibration travelling out of the machine into the duct, usually because there is no flexible joint between them.

The bang when it starts and stops

Press the bottom of an oil can and it pops. A big flat duct panel does exactly the same thing, twice a cycle.

Check 2

Take the pressure out of it first

When the blower starts, the pressure inside the duct comes up in a second. A large flat side bows outward under that pressure and snaps as it goes, and it snaps back when the fan stops. It is usually the big box right on top of the unit, because that is where the pressure is highest.

The filter. A clogged filter raises the pressure the blower has to work against, which makes the bang louder. New filter first, every time.

Every register open. Closed registers do the same thing, and people close them to save money and then wonder about the banging.

The return. Move whatever is in front of the return grille and take the furniture away from it. More breathing in means less pressure trying to burst the duct.

If it still bangs after all three, the cure is mechanical. A crease pressed diagonally across the flat panel, which the trade calls a cross break, or a length of angle screwed across it, stiffens the sheet so it cannot bow. New ductwork is made with those creases already in it, which is why it does not do this.

Ticking, rattling and whistling

Check 3

Three small noises with three small cures

Ticking and popping for a few minutes, then quiet. The metal is growing as warm air goes through it and rubbing where it passes a joist or a bracket. Harmless. The cure is finding the spot with a hand on the duct and slipping a strip of felt or foam between the metal and the timber.

Rattling at one register. The register itself. Tighten the two screws, and if the slats buzz, a strip of foam gasket behind the flange stops it. One roll of foam gasket does several registers.

Whistling at one register. Too much air through too small a gap. Open the lever fully, and if it still whistles, the damper further back is nearly shut or that branch is undersized for the amount of air being sent down it.

A rattle from inside the duct. Something loose in there. A damper blade that has come off its shaft is the usual one, and builders’ offcuts turn up more often than you would think.

A buzz that stops the instant the blower stops. Vibration from the machine travelling into the metal. The cure is a flexible canvas joint between the unit and the duct, which many systems never had fitted.

When it is simply too much air

A steady roar from everywhere, the whole time it runs, is usually not a fault. It is velocity, which means the system is pushing more air than the ducts were built to carry.

That happens when a bigger system was fitted to old ductwork, when the blower speed was raised during a repair and never put back, or when the ducts were undersized in the first place.

Closing registers to quieten it makes it worse, because the same air now goes through fewer openings faster. The real answers are a lower blower speed, which trades some airflow away, or a second return and bigger trunk, which is a proper job.

It is worth knowing which you have before spending anything, because the same roar with weak air at the far end of the house is a different story: what weak air at the registers means.

Questions people ask

Is the banging damaging anything?

Not really. It works the metal and it loosens joints over years, but it is a noise problem rather than a failure on its way. It is worth curing because it wakes people up, not because something is about to break.

Why is it worse in winter?

Heating moves hotter air, so the metal expands more and further, and cold dry air outside makes the whole house quieter to start with. The same ducts often go unnoticed all summer.

Can I seal ducts with duct tape?

Not the cloth kind, despite the name. ENERGY STAR says it is not long-lasting. Foil tape or duct mastic is what actually stays put.

It only ticks at night.

It almost certainly ticks all day too. The house is quiet at night and the setback temperature means longer runs, so both the noise and your attention go up together.

Would lining the ducts make it quieter?

Internal lining does cut noise, and it is usually done at the first few feet out of the unit. It is a job for a fitter rather than a weekend, and it does not cure a panel that is popping.

The noise is at the machine rather than in the ducts.

Then it belongs to the machine. Sort it by when it happens rather than what it sounds like: cooling noises or heating ones.