Water hammer is a pressure surge that occurs when a rapidly moving fluid (usually water) is suddenly stopped by a closed valve or other obstruction. The sudden change in momentum creates a shock wave that travels through the pipe, causing a loud banging noise and potentially damaging the plumbing system. Water hammer can be caused by a variety of factors, including: sudden closure of a faucet, a washing machine or dishwasher cycle ending abruptly, or a sudden change in water flow rate due to a malfunctioning valve or pump.