Knocking

We didn't use to get pipes knocking.
We run a Glow-worm Swiftflow combi boiler.
Suddenly now, when we turn the shower off (running off the combi boiler), we get a loud noise (maybe a knock).
Can anyone explain what may be the cause of this.
Thanks AD
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If you have lots of water at high pressure travelling fast through a pipe and then stop it dead, it has nowhere to go and sends a shock wave through the system in an attempt to dissipate the energy. It solved by installing a "shock arrestor" near the offending tap/valve. In the olden days, a short stub of pipe was left, full of air for the same reason. A shock arrestor is more reliable and hygenic. It is basically a tiny miniature expansion vessel, (sometimes spring loaded instead of using a air pressurised diaphragm).
It may have got worse recently because pressure is greater, a pipe clip has given up the ghost (after physically preventing the knock for some years), or someone with a faster technique has started using the tap/valve. A particular cause of new knocks is the installation of quarter turn taps/valves, which close much faster than a twist your arm off type.
Christian.
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You might have a washer going bad in the shower valve. If this just started and nothing has changed then look to the valve first. Does it happen when you turn off sink etc..?

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