Water Hammer

Water hammer is usually recognized by a banging or thumping in water lines. Although it might seem to be a smooth flow, the water inside the pipe actually churns and tumbles as it moves through. The normal sound of water moving through pipes is a steady, even sound. The best way to know what it sounds like is to go turn the bathtub water on full blast, then go to other rooms of the house and listen.
When water hammer occurs, a momentary abrupt pressure change of over 10 MPa may occur inside the piping.
This impact can severely jar piping, equipment or machinery housing, possibly resulting in damage not only to gaskets in junctions, but also to valve flanges or the valves themselves.

More Info about water hammer can be found on PlumbingMart.com