When I turn off the cold taps in my house there is a loud banging noise from behind the wall in the downstairs bathroom (which is where the cold water supply enters the house). This started happening some months ago now and I can't think of anything that happened plumbing-wise just before it started.
Anyway, I found that I can temporarily fix this 'water hammer' by turning off the water supply where it comes into the house and then draining the cold water system by running all the downstairs taps. When they run dry I turn the water back on at the main stop-tap and the banging noise is gone!
After a few days the banging starts to happen again – quietly at first but growing to the loud banging again after about a week.
What could this be?
EDITS:
My guess is that this has been caused by some bracket (or similar) that was holding a pipe behind a wooden wall coming loose. I'd much rather fix the cause and to install an arrestor in an appropriate place I'd have to mess with the wall anyway I think.
The only thing I can remember happening around the time this began was some very windy weather. When I heard the first bangs one night I assumed it might have been something banging in the neighbour's garage, which shares a wall with my bathroom.
Best Answer
Water hammer, no air quotes needed. Noise in pipes from the momentum of moving water, suddenly stopped. Draining the pipes is providing temporary air bubbles which cushion it.
Water hammer arrestors, either in the form of a few feet of capped vertical pipe at the end of the run to hold a more lasting bubble, or spring/plunger/diaphragm/piston units sold under that name will cure it. Shutting the water off more slowly will also cure it, where possible (automatic valves on washing machines and dishwashers are notorious for quick action causing water hammer.)
I have no association with any of the places these images come from:
Here's one where you can see it, so you might avoid opening walls if you did it this way:
You may have arrestors in place which have failed, given the "randomly started up a few months ago" part of this. Unfortunately you're probably going to have to open walls to find, replace, or add them.