Every time the clothes washer fills, you hear knocking and thumping in the wall or basement. Then it knocks again during the rinse cycle. What’s knocking?

Often it’s the water pipes knocking against the framing of your home. You may also hear a knock after the toilet fills or when you quickly turn off a faucet.

The knocking is caused by a phenomenon called water hammer. Water is heavy, and as it moves through pipes it carries lots of energy. When the water flow is stopped by a valve, energy in the flow shakes the pipes. This is a particular problem when a valve closes quickly – like an electrically operated valve (solenoid valve) in a washing machine.

What’s the fix? Add a water hammer arrester to the water line. A water hammer arrester provides an air cushion that compresses when water flow is quickly stopped. The air cushions the energy and prevents hammering.

Modern water hammer arresters are sealed so they should never lose the air cushion. Older arresters are just inverted, capped pipes that are supposedly filled with air. The old units can lose the air cushion, and then you need to drain the plumbing system to re-establish the air cushion.