Noisy pipes and water hammer: banging, knocking and what to check

A loud bang or a rattle of knocks when a tap is turned off, or when the washing machine stops filling, is one of the most common noises in a house. It is usually water hammer: moving water stopped suddenly. It is worth tracking down, because the shock goes through every joint each time it happens.

What water hammer is

Water moving through a pipe has weight and speed. If a valve shuts very quickly, that moving water has to stop almost at once. The energy turns into a pressure wave that runs back along the pipe and bounces. You hear it as a bang, a thud or a short series of knocks.

Three things make it worse:

• Fast-closing valves. Washing machine and dishwasher inlet valves snap shut electrically. Quarter-turn and lever taps can be shut in a fraction of a second. A cistern float valve can bounce open and shut as the water ripples.
• High pressure and fast flow. The harder the water is moving, the bigger the shock. Homes with high mains pressure hear it more.
• Loose pipes. A pipe that is not properly clipped can jump and knock against joists, floorboards or walls, which turns a small shock into a loud bang.

The Water Supply (Water Fittings) Regulations 1999, Schedule 2 require water fittings to be made and installed to resist damage from, among other things, "pressure surges", and say every water fitting shall "be adequately supported". They do not tell householders to fit any particular device, and we do not suggest any particular device is required.

Other pipe noises, and what they usually are

Not every noise is water hammer:

• Ticking or creaking as the heating warms up or cools down. Usually pipes expanding and contracting, rubbing against a notch in a joist or a tight clip.
• Humming or vibrating while a tap is running. Often a worn tap washer or valve, or a float valve, vibrating. See our guides to dripping taps and toilet fill and flush valves.
• A rumbling, kettle-like noise from the boiler. That is a boiler matter for a Gas Safe registered engineer. Do not try to investigate inside the boiler.
• Gurgling in radiators. Usually air in the heating, covered in our radiator bleeding guide.

What you can check

1. Find which fitting causes it

Have someone open and close each tap in turn, flush each toilet, and run the washing machine and dishwasher, while you listen. Note:

• which fitting sets it off;
• whether it happens when the fitting opens or shuts;
• roughly where the noise seems to come from.

If it only happens when the washing machine stops filling, the fast inlet valve is the likely trigger. Our washing machine plumbing guide covers the hoses and valves behind it.

If it happens after a toilet refills, look at the float valve. A slow, bouncing shut-off can cause repeated knocking.

2. Close taps more gently

It sounds obvious, but lever and quarter-turn taps can be slammed. If the noise goes when you close them slowly, the tap is the trigger.

3. Look for loose pipework

Where you can safely see the pipes, for example under sinks, in an airing cupboard, in a cellar or in a boarded loft:

• check that clips are present and holding the pipe firmly;
• look for pipes resting against each other, a joist or a board with no clip;
• look for scuff marks or dust lines that show where a pipe has been moving.

Adding or replacing a missing clip on an exposed run is a straightforward DIY job for many people. Keep to pipes you can reach without lifting floors over cables or standing on unboarded joists. Turn the water off if you are working near joints, and do not lever or bend pipes into place.

4. Check the isolation valves

A part-closed or failing isolation valve can whistle or chatter. Make sure the valve on the noisy appliance is fully open.

5. Think about pressure

If the noise is throughout the house and taps spit, the mains pressure may be high. Our guide to water pressure testing explains how pressure is checked, and our pressure reducing valves guide explains how high pressure is usually dealt with. A plumber is the right person to measure it and advise.

When to call a plumber

Call a plumber if:

• the banging is loud, frequent, or getting worse;
• you can see a joint weeping or water marks after a bang;
• the pipe that moves is under a floor, in a wall or in a ceiling;
• you think the pressure is high, or there is a pressure reducing valve that may need attention;
• a float valve or appliance valve needs replacing and you are not confident doing it.

A plumber may re-clip or re-route a run, change a valve for a slower-closing type, adjust or fit a pressure reducing valve, or fit a shock arrestor near a fast-closing valve. Which of these is right depends on the house.

Pipe clips at Wickes

We haven't tested these. They are examples, with what the retailer's page says.

We haven't tested this. Figures are as the maker states them (checked 13 November 2026).

Check your pipe size before you buy. Common domestic sizes are 15mm and 22mm, and a clip for one will not hold the other.

Is water hammer dangerous?

It is not usually an emergency, but repeated shocks put strain on joints, valves and appliance hoses. If you see a leak after a bang, turn off the water and call a plumber.

Why do my pipes bang when the washing machine fills?

Washing machine inlet valves close very quickly, which stops the water suddenly. Loose pipework nearby or high mains pressure makes the bang louder.

Do I need a water hammer arrestor?

Not necessarily. Many cases are cured by clipping loose pipes, closing taps more gently or dealing with high pressure. A plumber can tell you whether an arrestor would help in your house.