Noisy Water Pipes: Why They Bang and How to Stop It

Knocking water pipes almost always come down to water hammer, loose or missing pipe clips, or an airlock trapped in the system — and each one sounds slightly different. A sharp bang the instant a tap or appliance shuts off is water hammer; a rattling or knocking sound as water runs is usually a loose pipe hitting a joist or wall; a gurgling or hissing noise is trapped air.

• Water hammer — a shockwave through the pipe when a valve (tap, washing machine, toilet fill valve) closes suddenly
• Loose pipe clips — pipework that's no longer held firmly and knocks against joists, brickwork or floorboards
• Airlocks — trapped air in the system, common after plumbing work, a drained tank or a partial radiator bleed

Older London flats and terraces with rigid copper pipework tend to be noisier than newer plastic pipe installations, because copper transmits the shock of water hammer more readily.

That bang is water hammer — the moving column of water in the pipe has nowhere to go when a valve closes instantly, so the energy turns into a shockwave that slams against the nearest bend, joint or wall. It's most noticeable with modern fast-closing valves such as washing machine solenoids, toilet fill valves and mixer taps, which shut off far quicker than old-style screw taps ever did.

Mains water pressure that's higher than the system was designed for makes water hammer worse, which is one reason it's more common in parts of London on higher-pressure mains supplies.

Knocking that happens while your heating or hot water is running, rather than at the moment a tap shuts off, is usually a loose pipe clip letting the pipe expand, contract and knock against a joist or wall as it heats up and cools down. Copper and plastic pipe both expand slightly with heat, and a clip that's loosened, split or missing lets that movement turn into noise.

It can also be a sign of limescale narrowing the pipe or a failing valve causing turbulent flow, both of which are worth having checked if re-clipping accessible pipework doesn't stop it.

Gurgling, hissing or a soft knocking noise straight after any plumbing work, a drained header tank, or bleeding only some of your radiators is almost always an airlock — trapped air working its way through the pipework until it finds a way out. It's harmless to the system itself but can starve a radiator or tap of proper flow until it clears.

Bleeding every radiator in the house, starting furthest from the boiler and working towards it, usually clears an airlock within a few minutes per radiator.

Most banging or knocking pipes can be fixed in this order — start with the free checks before assuming you need a fitting or a plumber's visit.

1. Identify when the noise happens. Note whether the banging happens when a tap or washing machine valve shuts off (water hammer), when a radiator or the whole system is running (loose clips or airlock), or straight after any plumbing work (airlock).
2. Bleed your radiators. Use a radiator key to bleed each radiator in turn, starting with the one furthest from the boiler, until water rather than air comes out. This clears trapped air causing gurgling or knocking.
3. Check and tighten pipe clips. Look under sinks, in the airing cupboard and in any accessible loft space for pipe clips that have loosened or split — retighten or replace them so the pipe can't move and knock against joists or brickwork.
4. Fit a water hammer arrestor. If the bang happens right as a tap, toilet fill valve or washing machine solenoid valve shuts, fit a water hammer arrestor on that supply line to absorb the shockwave before it travels down the pipe.
5. Call a plumber if the noise persists. If pipes are still knocking after bleeding radiators, checking clips and fitting an arrestor, the cause is likely hidden behind a wall or floor, or linked to mains pressure — book a plumber to trace and fix it properly.

Call a plumber if pipes are still banging or knocking after bleeding radiators, tightening accessible clips and fitting an arrestor, because the noise is then most likely coming from pipework hidden behind a wall or under a floor, or from mains pressure that needs adjusting at the stopcock or a pressure reducing valve. RenoPlumb covers noisy pipe call-outs across East and Central London from our base on Commercial Road, E1, and we're Gas Safe registered if the noise turns out to be linked to your boiler or heating system rather than the cold water supply.

For anything that looks like an active leak rather than just noise — a burst pipe, or water actually escaping — treat it as an emergency instead and see our emergency plumber page, or call 07460 824073 now.

If the banging only happens when your heating fires up, it's worth ruling out a boiler pressure issue via our emergency boiler repair page, or a wider heating fault via heating engineer London.

The cost depends entirely on the cause — fitting a water hammer arrestor or re-clipping an accessible pipe is a short, low-effort job, while tracing a noise hidden behind a wall or floor takes longer. We give every job a free, no-obligation quote before any work starts, so you always know the cost upfront before asking. See our pricing page for the full breakdown of what affects the price.

Yes — RenoPlumb is based on Commercial Road, E1, and we fix noisy and banging pipes across Tower Hamlets, Hackney, Islington, Newham, Southwark, Canary Wharf, Shoreditch, Whitechapel, Bethnal Green and Mile End, as well as the wider East and Central London area. If you're nearby, see our plumber in Tower Hamlets page for local coverage details.