Where this starts
Water in a pipe has mass and it is moving, so stopping it quickly is like stopping anything else quickly: the momentum has to go somewhere. In a closed pipe it becomes a pressure spike that travels along the pipework and shakes it, which is the bang.
THE TRIGGER IS THE DIAGNOSIS, and it costs nothing to observe. Stand where you can hear it and have someone operate each tap, the washing machine and the toilet in turn. A bang at the moment a valve SHUTS is the pressure spike. A rattle or hum while water flows is something vibrating in the stream — a loose washer, a partly closed valve, a worn cartridge. A bang or a series of ticks with no water being drawn is almost always pipework expanding against structure as it heats.
APPLIANCES ARE OVER-REPRESENTED because they close their valves fast. A washing machine or dishwasher solenoid shuts in a fraction of the time a hand takes to turn a tap, so a house that bangs only when the machine fills is telling you the pipework has no way to absorb a fast closure — not that the machine is faulty.
WHERE the noise comes from matters as much as when. A bang heard throughout the house is the pressure wave travelling; a knock localised to one run under a floor is usually a pipe hitting something it can reach, which is a clipping problem rather than a pressure one.
The distinction that decides urgency: noise alone is a nuisance. Noise together with a joint that weeps, a pipe that has moved visibly, or a system that keeps losing pressure is worth acting on, because repeated shock loading works on joints.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A fast-closing valve stopping moving water
Most commonA single sharp bang at the moment something shuts, most often an appliance solenoid or a lever tap, and absent when the same tap is closed slowly.
How to check: Close the suspect tap slowly by hand. If a slow close is silent and a fast one bangs, the closure is the trigger.
Pipework not clipped enough, or clipped against something hard
Most commonA knock or rattle localised to one area, often under a floor or in a cupboard, sometimes felt through the floorboards as much as heard.
How to check: Whether the noise is local or general. A general bang is pressure; a local knock is a pipe hitting something.
Water pressure higher than the system is comfortable with
CommonBanging on several fittings rather than one, splashy taps, and appliances filling noticeably fast — commonly on a mains-fed system without a pressure-reducing valve.
How to check: Whether more than one outlet does it. One noisy fitting is that fitting; several is the supply.
A worn washer, cartridge or loose internal part
CommonA hum, chatter or vibration WHILE water runs rather than a bang when it stops, and usually traceable to one specific outlet or valve.
How to check: Whether the noise happens during flow or at shut-off. During flow is a component in the stream.
Pipes expanding against structure as they heat
CommonTicking, creaking or a single knock following the heating or hot water coming on, with no tap being used, and often on a predictable timetable.
How to check: Whether it happens with nothing running. A noise on a heating timetable is expansion, not water hammer.
A failed or waterlogged arrestor or expansion vessel
Less commonBanging that started after a period of quiet in a system that previously had none, sometimes alongside a pressure relief discharging or a gauge that swings.
How to check: Whether the noise is new in an unchanged system. Something that used to absorb the shock has stopped doing so.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Closing, running, or nothing — the three answers separate a pressure spike from a component in the flow from heating expansion, and nothing else does it as cheaply.
- 2
If slow is silent and fast bangs, the closure speed is the trigger and the fix is about absorbing it rather than about the tap.
- 3
A pressure wave travels and is heard across the house; a pipe knocking on a joist is local, and the two need completely different work.
- 4
It separates the plumbing from the heating circuit in one test, and heating noise has a schedule rather than a trigger.
- 5
One outlet points at that fitting's internals; several point at the supply pressure, which is a single change rather than several repairs.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where banging is accompanied by a joint that weeps, a pipe that has visibly moved, or a system losing pressure — repeated shock loading works on joints.
- Where a pressure relief valve or expansion vessel is involved, since those are safety components and their behaviour is a sign about the sealed system rather than about noise.
- Where the noise is on a gas or unvented hot water system, which is a controlled installation and not one to open up.
- Where the pipework runs under a finished floor or inside a wall and the remedy means opening it, which is worth planning rather than exploring.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.