Water hammer is a shuddering, knocking or banging noise in water pipes, caused by sudden changes in the speed of the water flowing in the pipes.
In the home, washing machines and dishwashers are often the culprits because they have electromechanical valves that open and close very quickly on their water supply lines. It can also occur in the supply lines for toilets.
- plumbing for general plumbing related questions. If you're asking a question about water hammer, it's probably appropriate to include this tag too.
- Wikipedia: water hammer