Why is my boiler making that noise? A sound-by-sound guide

Most boilers make some noise. A soft hum when the heating starts, or a click as it fires up, is perfectly normal. But new or louder noises are often an early sign that something isn't right.

The good news is that many boiler noises have simple causes, and some you can check yourself. Others need a Gas Safe registered engineer. At Go Assist, our boiler engineers hear about these noises every day once the heating goes back on, so here's what each sound usually means and what to do next.

Before you start: stay safe

Never remove the casing from your boiler. By law, only a Gas Safe registered engineer can work on gas appliances.

If you can smell gas, turn off the gas supply at the meter if it's safe to do so. Open the windows, don't use any electrical switches, and leave the property. Then call the National Gas Emergency Service on 0800 111 999.

1. Banging

A loud bang or thud from your boiler is one of the most common noises we hear about. It often happens when the boiler fires up, or the heating comes on.

The usual causes are:

• Limescale or sludge building up inside the boiler, which makes the water overheat and boil.
• Air trapped in the system.
• Low water pressure.
• A faulty pump that isn't moving water around the system properly.

What you can check: Look at the pressure gauge on the front of your boiler. When the heating is off, it should usually read between 1 and 1.5 bar. If it's lower, your boiler manual will explain how to top it up. You can also bleed your radiators to release any trapped air.

When to call an engineer: If the banging continues after you've checked the pressure and bled the radiators, book a boiler repair. Limescale and sludge need to be removed by a professional.

2. Kettling (rumbling or a low whistle)

Kettling sounds just like a kettle coming to the boil. It's a rumbling or whistling noise that builds as your boiler heats up.

It's usually caused by limescale or sludge on the heat exchanger, the part that heats your water. When the water can't flow freely, it overheats, creates steam bubbles, and makes that kettle-like sound.

Kettling is more common in hard water areas. If it's left, it makes your boiler work harder, use more energy, and wear out faster.

What you can check: You've already done the important part by noticing the issue. Now it's time to get a professional in to sort it.

When to call an engineer: Book an engineer to diagnose the issue and look at the heat exchanger. They may recommend cleaning the system to remove sludge. A magnetic boiler filter can also help stop sludge building up again.

3. Gurgling

A gurgling or bubbling noise usually means there's air in your heating system. You might also hear it in your radiators or pipes.

The usual causes are:

• Air trapped in the radiators or pipes.
• Low water pressure.
• A frozen condensate pipe, if the weather has been very cold.

What you can check: Bleed your radiators, then check the boiler pressure again, because bleeding can make it drop. In freezing weather, check the white plastic condensate pipe that runs outside. Our guide to frozen condensate pipes explains how to thaw it safely.

When to call an engineer: If the gurgling keeps coming back, there may be a leak letting air into the system, or a problem with the pump.

4. Whistling or screeching

A high-pitched whistle usually means water isn't flowing through the boiler as it should.

The usual causes are:

• A blockage in the heat exchanger from limescale or sludge.
• Low water pressure.
• A valve that's partly closed or sticking.

What you can check: Check the pressure gauge. Make sure the valves on your radiators are open.

When to call an engineer: If the pressure is fine and the whistling continues, book an engineer. A blocked heat exchanger can cause your boiler to overheat and shut down.

5. Humming or vibrating

A gentle hum is normal for most boilers. A loud hum, buzz, or vibration is not.

The usual causes are:

• The pump running at too high a speed.
• Loose parts or pipe fixings inside or around the boiler.
• A fan that's starting to wear out.

What you can check: Look for any loose pipe clips near the boiler. Don't open the boiler casing.

When to call an engineer: An engineer can adjust the pump speed or replace a worn fan. Both are usually quick jobs.

6. Clicking

A few clicks when your boiler starts are normal. That's the ignition lighting the burner.

Repeated clicking with no heat is different. It usually means the boiler is trying to light but can't.

The usual causes are:

• A faulty ignition electrode.
• A problem with the gas supply.
• A faulty thermostat or valve.

What you can check: Make sure your other gas appliances, such as your hob, are working. If they aren't, contact your gas supplier. Check that your thermostat is set high enough and has working batteries.

When to call an engineer: If your gas supply is fine and the clicking continues, book a repair. Ignition faults need a Gas Safe engineer.

7. Tapping or ticking in the pipes

Light tapping or ticking as the heating warms up is usually nothing to worry about. Metal pipes expand as they heat and contract as they cool.

If the tapping is loud, or happens with banging or gurgling, it may point to one of the problems above.

Signs of a more serious boiler fault

Some warning signs need attention straight away. Turn your boiler off and call the National Gas Emergency Service on 0800 111 999 if you notice:

• a yellow or orange flame instead of a crisp blue one
• black or brown sooty marks on or around the boiler
• more condensation than usual on nearby windows
• headaches, dizziness or nausea that ease when you leave the house.

These can be signs of a carbon monoxide leak, which can be fatal. You can't see, smell or taste carbon monoxide, so don't wait. Turn off your boiler, open the windows, get everyone out of the property and call the National Gas Emergency Service on 0800 111 999 straight away. If anyone feels unwell, get medical help. Make sure you have a working carbon monoxide alarm near your boiler.

How to keep your boiler working correctly

Most of these noises build up over time. The best way to prevent them is an annual boiler service. An engineer will check the pressure, clean key parts and spot small problems before they turn into breakdowns.

Between services, check the pressure every few weeks and bleed your radiators at the start of each winter.

Boiler still making a noise?

If your boiler is banging, whistling or clicking and you're not sure why, our Gas Safe registered engineers can find the cause and fix it, often the same day or the next. Book online in a couple of minutes or call us on 0333 733 1234.