Why it happens
Water is heavy and does not compress. When a fast-closing valve — a washing machine, dishwasher, ice maker or single-handle faucet — shuts off flowing water, the water’s momentum has nowhere to go and becomes a pressure spike.
What makes it worse
- High household water pressure
- Long straight runs of pipe
- Pipes that are not secured, so they move and knock
- Old air chambers that have filled with water
How it is fixed
Water hammer arrestors — small sealed cushions installed near the fixture — absorb the spike. Reducing high house pressure with a regulator and strapping loose pipes also helps.
Why not to ignore it
Repeated pressure spikes strain fittings, appliance valves and supply lines. A noise that seems harmless can shorten the life of the parts it shakes.