Water hammer
Water moving through a pipe carries momentum. When a valve closes quickly — a washing machine, dishwasher or single-handle faucet — the water stops abruptly and the shock travels back through the pipes as a bang.
High water pressure makes it worse, and so do air chambers that have filled with water over time.
Other noises
- Rattling or knocking while water runs: pipes not secured to framing
- Ticking as hot water flows: pipes expanding and rubbing against wood
- Whistling or squealing: a worn valve or a regulator under strain
What you can safely check
Note which fixture causes the bang. If the house has an old air chamber, turning off the main, opening the highest and lowest faucets to drain the system, and then refilling it can restore the air cushion for a while.
When professional help makes sense
Persistent water hammer stresses joints and appliance valves. A plumber can check the house pressure, fit water hammer arrestors at the offending fixtures and secure loose runs of pipe.