What to Do When Your Pipes Bang and Knock

That bang when the washing machine shuts off is called water hammer — moving water stops dead and the shock wave rattles the pipes. Left alone, it can loosen fittings over the years.

Calm it down

• Check your home's water pressure with a gauge on an outside faucet. Above 80 psi is too high and causes all kinds of plumbing grief — a pressure regulator is the fix.
• Make sure pipes are strapped snug in the attic or crawl space. A loose pipe bangs because it can move.
• Drain-down trick: shut off the main, open all faucets to drain the system, then close them and turn the water back on. This refills the air chambers built into many systems.

If the banging persists, a plumber can add water hammer arrestors at the worst appliances. Don't ignore it — the noise is a symptom, not the disease.