water hammer

  1. the hammering sound caused in a pipe containing water when live steam is passed through it
  2. the thump of water in a pipe, caused by an air lock, when a faucet is suddenly closed

See water hammer in American Heritage Dictionary 4

noun
  1. A banging noise heard in a water pipe following an abrupt alteration of the flow with resultant pressure surges.
  2. A banging noise in steam pipes, caused by steam bubbles entering a cold pipe partially filled with water.
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