Water Hammer and Noisy Pipes

The short version

A sharp bang when a tap, toilet or appliance valve shuts is water hammer: moving water stopping suddenly and slamming into the closed valve. Knocking as hot water runs is usually pipes expanding against tight straps. A hum or whistle points to high pressure or a worn valve. Each has a different fix, so start by naming the noise and when it happens.

The routine, in order

1. Note the timing. Does the noise happen when the washing machine or dishwasher stops filling? When a toilet finishes? When hot water runs for a minute?
2. Check the pressure. Screw a gauge onto an outdoor tap or laundry faucet. Very high pressure makes hammer much worse.
3. Look at the straps. In unfinished basements, check whether pipes are loose and hitting joists, or clamped so tightly they cannot expand.
4. Drain the system. In older homes with air chambers, shutting the main and draining from the lowest tap can restore the cushion of air in the chambers.
5. Listen at the water heater. Popping and rumbling from a tank is usually sediment, not pipe noise. See our water heater flushing pamphlet.

Mistakes to avoid

• Ignoring loud hammer. Repeated shocks stress joints, valves and appliance hoses.
• Raising pressure to fix a weak shower, then wondering why the pipes started banging.
• Strapping hot water pipes rigidly. They need a little room to move as they warm.
• Confusing sediment noise in a water heater with pipe noise.
• Adding foam insulation over a loose pipe and calling it fixed. The noise may stop; the movement will not.

Milton's hard water adds a twist. Scale builds up in fill valves and cartridges and can make them close more abruptly or chatter as they wear. A hard water problem sometimes shows up as a noise problem first.

When to ring the shop

Call when the bang is loud or getting worse, when pressure tests high, or when the noise sits behind finished walls and ceilings where you cannot reach the pipe. Fixes include water hammer arrestors near quick-closing valves, a new or reset pressure-reducing valve, better pipe supports and replacing chattering valves. These are usually modest jobs. Tell us when the sound happens and which fixtures trigger it, and we can often narrow the cause before we arrive. A short video with sound, recorded on your phone, is surprisingly useful./p>

A noisy drain is a different story. Gurgling from sinks or toilets points to venting or a partial blockage, and a sewer camera inspection can settle that if the noise comes with slow drains throughout the house.