Heat Pump Noise: Which Ones Are Meant to Happen

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

Heat Pump Noise: Which Ones Are Meant to Happen

Short answer

A heat pump in winter makes noises an air conditioner never makes, and most of what gets reported is the defrost cycle doing exactly what it was built to do. Four noises mean switch it off tonight. Here is how to tell those from the whoosh, the gurgle and the steam.

A lot of heat pump noise worries start in the first cold week of the year, and often nothing is wrong.

That is not a brush off. A heat pump does something no air conditioner and no furnace does: it runs in freezing weather, it grows ice on purpose, and every so often it throws that ice off with a bang. Nobody tells owners that in advance.

Before you start

What to carry: a torch, and ten minutes standing next to the outdoor unit. Nothing comes apart.

Which machine is this. A heat pump is electric and has no flame, no flue and no gas. If yours burns something, you are on the wrong page: what each furnace noise means.

Do it twice. Once while it is running normally, and once during a cold morning when it is working hard. They sound completely different and that is the whole point.

Four noises that mean switch it off, tonight. Metal screaming or grinding. A bearing or a fan hitting something. Every extra minute makes the repair bigger.

A loud hum with nothing turning. A motor being fed power and unable to move is heating up in a way that ends badly.

A bang or a clatter, and then it stops working. Something has come loose inside.

Any noise at all with a burning smell. Off at the breaker and leave it off. What the smell itself is telling you: heat pump smells.

And never chip ice off with anything. A screwdriver through a coil turns a free problem into a refrigerant leak and a big repair bill.

The defrost cycle, which is most of it

Every one of these is meant to happen, in this order, a few times a day in cold weather. It lasts five to fifteen minutes and then the machine goes quiet again.

Check 1

Work out whether you are just hearing a defrost

In cold damp weather the outdoor coil grows frost, so the machine reverses itself for a few minutes and melts it off. While that happens it sounds alarming.

The fan stops dead, first. Sudden silence outside, often mistaken for a breakdown.

Then a loud whoosh or a heavy clunk. That is the reversing valve swapping the flow round. On some units it is genuinely startling, and Lennox and Trane both describe it as a normal part of defrost.

Hissing, gurgling and bubbling. Refrigerant moving in the other direction, plus water running off the coil.

Steam. White vapour rolling off the unit looks exactly like smoke and is not. There is no fire in a heat pump to make smoke with.

Then the fan restarts, sometimes with a thump. Five to fifteen minutes from start to finish, a few times a day.

The test. If the noise only happens in cold weather, lasts a few minutes, and the machine carries on afterwards, it is defrost. If it happens all year or does not stop, read on. If the ice never clears at all, that is a different fault: when the ice is not normal.

Noises from the unit outside

Which end is making it narrows the list before you have touched anything. Outdoors is the compressor, the fan and the valve. Indoors is the blower and the ducts.

Check 2

Name the outdoor noise

A loud buzzing or humming that comes and goes. The contactor, which is the relay that switches the compressor on. A worn one buzzes and eventually sticks, and a stuck one is why some units never shut off.

A rattle you can stop by leaning on the cabinet. A loose panel screw. Free, and it is common on a unit that has done a few winters.

A hum on starting that clears after a second, getting worse each year. The start capacitor giving up. Cheap now, expensive if it takes the compressor with it: why this is the part that fails most.

A jet engine on startup that settles down. Normal on many newer units, which ramp the fan and compressor up. Loud is not the same as wrong.

A regular tapping or slapping in time with the fan. Something in the fan path. A leaf, a twig, or a fan blade that has bent and is touching a guard. Power off before you look.

A deep vibration you feel through the wall. The pad has settled or the mounting feet have gone hard. It is fixed with rubber feet or by levelling the slab, not by a new machine.

Silence, when it should be running. That is a different page altogether: nothing at all outside.

Noises from inside the house

Check 3

Name the indoor noise

A whistle at the vents or a roar that has grown over months. Airflow, and usually a filter. It is the cheapest thing on this page and the one most often ignored.

A click a second or two before it starts, and another as it stops. The relays on the board. Meant to happen.

Banging or popping in the ducts at start and stop. The metal itself flexing, not the heat pump: why ductwork bangs and how it is quietened.

A squeal that lasts the whole run. The indoor blower. A dry bearing or a belt on an older air handler, and it gets worse every week.

A rumble for a second or two after it shuts down. The blower coasting, plus air settling. Normal.

Water running or trickling. In heating that is usually melt water outside finding the drain. Indoors, in cooling season, a trickle is worth following up.

Questions people ask

Mine bangs loudly two or three times a day in winter.

Almost certainly the reversing valve at the start of a defrost. Time it against the weather for a couple of days. Cold and damp mornings should be the worst, and a mild dry day should be quiet.

It is much louder than my old air conditioner.

It is doing more. A heat pump runs all winter as well as all summer, and it works hardest on the coldest days, which are also the days the garden is silent and every sound carries.

Steam is pouring off it. Should I be worried?

No. There is nothing burning in a heat pump. That is ice melting off the coil during defrost and it is the sign the machine is doing its job.

It hums loudly but nothing moves.

Switch it off at the breaker rather than leaving it. That is usually a capacitor or a seized motor, and leaving power on a motor that cannot turn is how a cheap part becomes an expensive one.

The noise only happens when it switches between heating and cooling.

Same valve, same whoosh. It is the one moving part that changes the whole direction of the system, and a noise as it moves is expected.

Can anything be done about how loud it is normally?

Sometimes. Levelling the pad, fitting rubber feet, clearing at least two feet round the unit and moving anything that rattles nearby all help. A sound blanket on the compressor is a real product, and it must be one made for the model rather than anything wrapped round it.

Sources: Lennox "Understanding the Heat Pump Defrost Cycle" (whoosh, steam, backup heat, 5 to 15 minutes), Trane "What Is the Heat Pump Defrost Cycle?" (swooshing, the outdoor fan stops, steam) and "Is This Normal? HVAC Smells and Sounds, Explained" (hum from the capacitor, contactor or fan motor, loud starts), Carrier 25HCE heat pump installation instructions (reversing valve, outdoor fan stops in defrost) and the Fujitsu ASU9RL and ASU12RL operating manual (sizzling and steam during defrost)