A water hammer occurs when a flow of water is forced to abruptly stop of change direction. Water hammers generally occur when a valve in a pipeline is suddenly closed off to a flow of water.
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Yeah, HowStuffWorks says water hammer "occurs
when you shut off the water suddenly and the fast-moving water rushing
through the pipe is brought to a quick halt, creating a sort of shock
wave and a hammering noise. Plumbing that's properly installed has air
chambers, or cushions, that compress when the shock wave hits,
softening the blow and preventing this hammering."

So if you're experiencing water hammer with your plumbing, it should probably be looked into. Article below has some advice...

Source(s):
http://home.howstuffworks.com/how-to-fix-pipes5.htm

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