Whenever we turn off the hot water tap in either the bathroom or kitchen we get a hammering noise unless we turn off the taps slowly. I thought it might be an airlock (as suggested by several people) but a plumber tried the taps and said we needed to strip off all the tiles in the bathroom to find a loose pipe !!
The plumber hasn't the greatest reputation in our part of the world and was wondering if anyone else had a similar problem - and if it was was easily solved!
When I had one I simply connected a hose from the cold tap which was at mains pressure, to the hot tap which was at a lower presuure, and forced water back up the hot system until the overflow pipe had been pouring for a while. Seemed to clear it.
OG's tip is something I've done a few times.......... that, and sticking my mouth over the hot tap and blowing :o)
Loose pipe maybe, but if not airlock, then water hammer. It's a simple job to fit an arrestor like this................
Be careful if putting the cold tap on the hot. If you have a tank upstairs or in the loft the flow into it via the hot will be under pressure and will fill the tank faster than the overflow pipe can empty it.
Look .. It's water hammer causing a pipe to knock somewhere.
Also, if there is a delay before knocking while any water header tank is filling, try replacing tank ballvalve, replacing with a quiet fill valve (which damps down fill rate) or turning down mains stopcock to 1-1 1/2 turns open and retry.