Why Is My Boiler Making Noises?

18 July 2023

If your boiler starts making unusual noises, there’s a chance there could be a problem with it. That could simply mean that your boiler is not running as efficiently as possible. 

Or it could mean that a component has failed – or is about to fail – and a boiler repair is needed immediately. In this article, we’ll look at what noises you should expect your boiler to make when it’s in good working order. And we’ll also go through some of the more common sounds that a boiler can make when something is amiss.

What noises should a boiler make?

Boilers are not meant to be completely silent. There’s a lot going on in there, with pumps, flowing liquids and gases, and switches and valves operating as it goes through its routine cycles. They do come with some sound insulation in the case, but not enough to prevent every sound from escaping.

If your boiler is hidden away under the stairs, you might rarely hear it, but if it’s mounted on a wall in the kitchen, as many combi boilers are, you’ll hear it frequently, and you might even need to turn your radio up a bit if it’s heating up water.

When your boiler is new, or if it has recently been serviced, take a minute or two to listen to it while you put it through its various functions, such as operating central heating, heating water in the cylinder or heating water directly. 

You should hear the whirring of pumps, and you’ll probably also hear some slight gurgling noises that you’d expect to hear when water is being moved, especially straight after the boiler has been started up.

Even when it’s not boiling any water, there might be a faint sound of electronics. And when it’s cooling down after being run, again, you might hear some gentle clicking, just as you would with a car engine after you’ve parked up. This is all normal.

There are other sounds, however, that could indicate that your boiler needs attention.


You might read the word “kettling” when you’re looking into boiler noises, but in truth, it isn’t really a specific sound. Just as a kettle can make many different sounds as it turns cold water into boiling water and then cools down again, so can a boiler as fluids do different things inside it. 

That’s why kettling can often mean whistling, but can also mean rumbling, popping or tapping. Thay are all signs that there’s excessive turbulence or some sort of restriction inside the boiler, however. In general, kettling is caused by bubbles of steam inside the water. 

Although it’s called a “boiler”, it isn’t meant to heat water to boiling point, 100 °C (212 °F). Water only really reaches about 70 °C in normal use, and some people will have their boilers turned down to 60 °C.

Water is pumped past the flames of the burner at just the right speed to make sure it reaches the desired temperature. But if it is slowed down for some reason, it will linger over the flame, and could easily reach boiling point. Suddenly it’s a gas, and boilers aren’t designed to deal with gases. These unusual sounds are often caused by steam passing through the pipework and pump. 

So what can slow down the flow of water? There are two prime suspects in any kettling case.

First, there’s a build-up of limescale in the heat exchanger, or along the pipework. The heat exchanger is where the circulating hot water warms up the mains water that will come out of the tap or fill the cylinder. It’s full of small gaps to make the exchange more efficient. 

These gaps can easily clog up with limescale, especially if you live in a hard water area. There are measures you can take to prevent or lessen it, but for now, the boiler will need to be cleaned or have some components replaced.

Another cause is a faulty pump or thermistor. A pump that is only working at half pace will push water too slowly over the flame, and it will overheat. The thermistor regulates the temperature, telling the boiler to increase or reduce the heat. If that’s giving false readings, it might think the water is at 70 °C when in fact it’s at 100 °C.


A gurgling sound is also caused by a gas, and although it might be steam, it is more likely to be air. Air can build up in even a closed loop. You’ve probably seen bubbles form around a boat’s propellor, or even in a glass of water left to stand. It can enter the system through leaks or it can be left over from installation.

An air problem is usually cumulative – that is, it happens slowly over time, but eventually it reaches a point where there’s enough of it to make a gurgling sound, like blowing into a milkshake through a straw. 

One thing you can try is to bleed a radiator. The water inlet and outlet are at the bottom, so air will collect at the top. Releasing the air with a radiator key can get rid of a lot of bubbles, although you might have to top up the water pressure after you’ve done it.

Mechanical failure

Finally, some boiler sounds might have nothing to do with the water, and could simply be a component malfunctioning. 

The pump is usually running for several hours a day, and potentially all day long in the depths of winter, so it’s no surprise that they will wear out eventually. 

The motor struggling, or certain components getting worn down, can all cause the pump to stop operating as intended, and that’s bound to change the noise it makes. 

Other moving parts include valves and switches, which control where the water travels, how fast it goes and when it’s on or off. If these are starting to fail, you might hear different noises to what is normal. 

Call in the experts

If your boiler suddenly starts making unusual noises, it’s best to have a gas safe engineer pop in and have a look at it. It could require a simple solution like a bleed, or perhaps a power flush. 

The engineer might be able to put devices such as filters in place to prevent the issue from recurring. If a component is beginning to fail, you’ll be glad you called. You might have just saved yourself a lot of money and shivering by catching it early and having it repaired.