This “normal” noise means trouble later engineers quietly watch

You’re making tea, the boiler’s just kicked in, and somewhere behind a wall a pipe goes bang like a cupboard door catching the wind. That “normal” noise is often water hammer, and it matters because the repeated jolts drive pipe stress into joints, valves, and fixings long after the drama of the bang has faded. Engineers quietly clock it because it’s less about the one scare and more about the thousands of small shocks that follow.

It’s easy to shrug off because the water still runs and nothing looks wrong. But plumbing failures rarely announce themselves with a tidy countdown; they arrive as a weep under the floor, a loose tap, a valve that suddenly won’t shut, or a radiator that starts to sound like it has opinions.

The noise people normalise (and why it’s not just “pipes settling”)

Water hammer is a pressure surge: water is moving, something closes quickly, and the moving column of water has to stop now. That sudden stop sends a shock wave through the pipework. You hear it as a sharp bang, a thud, or a machine-gun rattle, often after a tap flicks shut, a washing machine finishes a fill, or a toilet valve snaps closed.

“Pipes expanding” tends to be a creak, tick, or gentle knock as hot pipework warms and rubs against timber. Water hammer is different. It has that slammed-door quality, and it usually lines up with a valve action - on/off, fill/stop - rather than a slow heat-up.

What’s actually happening under the bonnet

A lot of home systems are designed for smooth flow, not abrupt stop-starts. Modern appliances, combi boilers, mixer taps, and solenoid valves can close fast enough to create the perfect conditions for a surge. In older properties, pipe runs might be longer, clips might be sparse, and bits of pipe might be loosely resting on joists where any jolt gets amplified like a drum.

The key detail engineers care about isn’t just the sound - it’s the repeated mechanical load. Every bang is a little hit to joints and components. Over time, that’s how “fine for years” becomes “why is the ceiling stained?”

The places it tends to start (so you can catch it early)

If you’re trying to pinpoint it, think in terms of triggers rather than rooms. The loudest noise isn’t always where the surge originates.

Common culprits include:

• Washing machines and dishwashers (fast-closing inlet valves)
• Toilets (fill valves that shut abruptly or are set too high)
• Quarter-turn lever taps (quick shut-off)
• Combi boilers and zone valves (rapid changes in flow)
• High mains pressure coming into the property

If the bang happens only when one fitting operates, that’s a clue. If it happens across several, you may be looking at pressure and system-wide flow behaviour rather than one naughty tap.

Why engineers quietly watch it: the slow damage pattern

The first risk is obvious: the bang can loosen clips, shake pipes against masonry, and make fittings rattle. The second risk is the boring one that costs money later: small movement at threaded joints, compression fittings, and valves, repeated until a seal stops sealing.

Water hammer also makes the whole system feel “hard”. You might notice:

• Taps that judder when shutting off
  
• A buzzing or chattering valve during fills
  
• Pipes that shake behind a cupboard when an appliance draws water
  
• A toilet that finishes filling with a single loud knock
  

None of these guarantee a failure tomorrow. They’re just the system telling you it’s being knocked around more than it needs to be.

What you can do without ripping out your kitchen

You don’t need a full replumb to reduce the problem. The goal is to soften the stop, reduce the speed/pressure, or give the surge somewhere to go.

1) Check pressure first (because high pressure makes everything louder)

If your mains pressure is high, water hammer is easier to trigger and harder to ignore. A plumber may suggest a pressure reducing valve (PRV) if it’s excessive. As a homeowner, you can at least notice the signs: aggressive tap flow, frequent banging, and appliances that seem “snappy”.

2) Secure and cushion pipework where you can reach it

Loose pipes amplify the noise and the stress. If you can access under a sink, behind a toilet, or in a service cupboard, look for pipe runs that can move. Proper clips and a bit of isolation from timber can make a surprising difference.

3) Slow down the offenders

Some valves can be adjusted to close more gently, and some taps benefit from a slightly slower operating style. Toilets are a big one: a fill valve set correctly and in good condition often reduces the end-of-fill knock.

4) Add a place for the surge to go

This is where water hammer arrestors (or air chambers, properly designed) come in. They act like a cushion, absorbing the pressure spike. They’re commonly fitted near appliances with quick-closing valves. It’s not glamorous kit, but it’s the kind that stops a system from punching itself in the joints.

A quick “is this the bad kind of knock?” checklist

If you want a simple read on it, use timing and repetition:

• Right after a tap snaps shut? Likely water hammer.
  
• Only when hot water starts/finishes heating? More likely expansion noises.
  
• Happens when the washing machine stops filling? Classic trigger.
  
• Getting louder or more frequent over weeks? Worth acting on now, not later.
  

One bang that happens twice a year is an annoyance. A bang you can reproduce on command is a maintenance issue waiting for a convenient moment to become a leak.

When it’s time to call someone in

If you can’t safely access the pipework, or if the noise is paired with pressure oddities (taps spitting, valves chattering, toilets refilling randomly), get a plumber to assess it. Engineers don’t “watch” water hammer because it’s mysterious - they watch it because it’s predictable: repeated shock loads tend to show up later as small failures in the weakest point of the run.

You don’t need to panic. You just don’t need to normalise it, either.