Water hammer

When a washing machine valve or single-lever faucet slams shut, the moving water has nowhere to go and the shock wave bangs the pipes. Over time it can loosen fittings and damage appliance valves. Hammer arrestors, small shock absorbers installed near the offending fixture, usually quiet it down; chronic hammer can also point to high water pressure.

Why the bang in your walls is mechanical stress, not a quirk

That thud is a genuine pressure spike — water moving through a pipe carries momentum, and when a solenoid valve on a washer or dishwasher snaps shut in milliseconds, the moving column slams to a stop and the shock wave can multiply the line pressure several times over for an instant. Repeated thousands of times, those spikes loosen fittings, fatigue solder joints, stress appliance inlet valves, and rattle poorly secured pipe against framing until something starts to seep. The noise is the harmless part; the fatigue it announces is what eventually shows up as a leak inside a wall.

Trace the source, then check the pressure

Note which event triggers the bang — washer fill cycles and dishwasher valves are the usual culprits for a single sharp thud, and an arrestor installed at that appliance usually ends it. Hammer at many fixtures, or that appeared across the house at once, points at a system-wide cause: check your water pressure with a hose-bib gauge, because chronic high pressure amplifies every shock, and a home over 80 PSI needs a pressure-reducing valve, not just arrestors. A machine-gun rattle when a faucet runs is a different animal — usually a loose washer or worn cartridge in that fixture rather than true hammer.