Water hammer is a very loud banging, knocking or hammering noise in the pipes that occurs when the flow is suddenly turned off. It is caused by a pressure or shock wave that travels faster than the speed of sound through the pipes, brought on by a sudden stop in the velocity of the water, or a change in the direction. It's also been described as a rumbling, shaking vibration in the pipes.
You might hear water hammer when the clothes washer stops filling, the sprinkler system shuts off or shifts to another zone, when the dishwasher changes wash cycles, or when a faucet is turned off suddenly. It exerts very great instantaneous pressures that can reach excesses of 1000 psi and over time can potentially damage the system by weakening joints and valves causing leaks or even ruptures in the pipes