Knocking noise in pipes

[frane]

When I flush the toilet I get a loud knocking noise in the pipes while it refills. It has only just started to do this. What could be causing the problem and what can I do to stop it?

[norfolk boy]

Sounds like its time to change the washer in your ball valve or for a quick fix turn the isolating valve to half flow.

[manics_]

It sounds like an air lock in your cold water pipe,it may just need bleeding. Althoughit could be as norfolk boy says and be the ball valve.

[quattro]

It could be water hammer if your water pressure is too high try turning down your main stop cock

[frane]

Thanks for the responses guys. I didn't know what an isolation valve was or how to bleed the pipes, so I started off by turning down the main stop cock. This sorted out the problem. I tried turning the pressure up again and the knocking didn't come back - although there is a slight single knock as the cistern stops filling. It's MUCH better though. Thanks! P.S I've also now found out what the isolation valve is! But if I turn that down, the refilling seems to get noisier so I've left it fully on.

[Andrew C]

The knocking is caused by the pressure of the water suddenly stopping and then boucing back through the pipes. The knock soudn is pipes touching, if you can seperate the pipes or insulate them form each other then you will solve the problem permanently. I had the same problem when the cold tap would suddenly be turned off. Turning the main stop tap down helped but now I have the tap fully open and no noise. Try to isolate where the knock sound comes from and that will be the problem area to look at first.