A sharp bang is often caused by a sudden stop in flow
Water hammer happens when moving water is stopped quickly and the pressure wave travels through the pipework. Lever taps, washing-machine valves and some modern appliances can close very fast, making the effect more noticeable than an old-fashioned screw-down tap. The bang may seem to come from a wall nowhere near the outlet because the pipe moves where it is least well supported. A single knock at shut-off is different from continuous rattling or a droning sound, so describe the noise as accurately as you can.
Loose pipes can make a modest pressure change sound dramatic
Pipework that is not clipped securely can strike timber, plasterboard or another pipe when the pressure changes. Older installations sometimes have long unsupported runs hidden beneath floors. The water hammer creates the movement, but the loose pipe creates much of the noise. If the banging began after a kitchen or bathroom alteration, new fast-closing fittings may have exposed an existing weakness. The remedy may therefore involve both controlling the pressure shock and securing accessible pipework.
Pressure level can make the problem worse
High incoming pressure increases the energy in the system and can make shut-off noise more severe. A plumber may check the supply pressure rather than assuming an arrestor is the answer. Pressure-reducing valves can be used where appropriate, but they should be selected and set for the installation rather than fitted simply because the pipes are noisy. In some cases, a faulty valve or worn tap mechanism chatters instead of producing true water hammer, so listening to when the sound occurs matters.
What you can observe safely
Try different outlets one at a time and note which action causes the bang. If a washing machine or dishwasher is responsible, the noise may occur during its fill cycle rather than when you operate a tap. Compare slow and fast closing of a manual tap; if the noise only happens when the flow is stopped abruptly, that supports the water-hammer explanation. Do not start lifting floors just to find the pipe. A clear pattern of which valve triggers the sound gives a plumber a much better starting point.
Possible remedies
The right fix depends on the cause. Accessible loose pipes may be clipped or cushioned. Excessive pressure can be reduced. In systems where fast solenoid valves create unavoidable shocks, a correctly placed water-hammer arrestor may help. A defective tap or valve may simply need repair. If the noise comes from heating pipework rather than fresh water, thermal expansion and pipe rubbing become more likely causes. Treating every bang as the same problem can lead to unnecessary fittings being added without curing the noise.
When to get it looked at
Occasional mild noise is common in older homes, but repeated violent banging is worth investigating because movement can stress joints and valves. Tell the plumber which fixture triggers it, whether the sound is one bang or a series, and whether it happens on hot water, cold water or an appliance connection. Mention any recent plumbing changes. A short recording can help if the noise is difficult to reproduce during a visit. The more precise the description, the less time is spent chasing unrelated pipework.
A quieter system should still work normally
The aim is not merely to silence the loudest bang by restricting the water until the taps become frustrating to use. Any pressure adjustment should leave the house with adequate flow, and pipe clips should support the pipe without preventing normal thermal movement. If an arrestor is fitted, ask why that location was chosen and which valve is creating the shock. After the work, operate the troublesome tap or appliance several times at normal speed. Listen in the room where the noise was previously worst. If one bang has disappeared but a different rattle remains, there may still be loose pipework elsewhere. Water-hammer repairs are most convincing when the original trigger can be reproduced before and after the change.
This page is general guidance about noisy pipes water hammer; the cause in an individual home still needs to be established from the actual installation.