Water hammer is a specific plumbing noise, not a generic name for pipe clatter. It occurs when you shut off fast-moving water suddenly, bringing it to a quick halt and creating a sort of shock wave and a hammering noise. Fix “water hammer” by draining the plumbing system. Open the uppermost faucet (or the one furthest from the water meter) and the lowest (or closest to the meter) and allow the water to flow to a lower-level sink or floor drain. Draining the system restores air to air chambers. Close the lowest faucet and refill the system. For more detailed information see this article.