toilet flush?

water hammer its the shock wave from the high pressure water suddenly stopping.

as said you can reduce it by slightly shutting the isolation valve.
but turning it to far will restrict the flow not the pressure.

the proper way to cure water hammer is a arrestor fitted in the pipework.

hi-spec plumbing & heating (essex)
August 2009

Try reducing water pressure at the shut off valve, probably too fierce and causing the valve to jump/vibrate.

Bob
August 2009