It's actually called water hammer.
It occurs when you abruptly start or stop the flow of water in a pipe. It's caused by the pipe's movement, and its cure is to add a pipe support at a location that will oppose the direction of motion. First check for a support that may have loosened or developed a gap between the support and the pipe. If it's behind a wall it's obviously more difficult to fix. First try clamping the pipe where it penetrates the wall. Just a wooden wedge between the pipe and wall opening might be enough. Not to worry though, it's noisey but only in severe cases is it a threat of damageing the pipes and leaking.
Shazaam!
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