I regularly get a knocking noise from my mains cold water pipe. It isn't loud, it is a tap, tap, tap sound rapidly repeating. It continues for maybe 20-30 seconds and then stops; it then repeats after anything from 1 - 30 minutes and this can contiune for some time. Then suddenly you don't hear it for hours. Often the tapping sound is relatively early morning 6 - 10:00. The tapping starts and stop for no apparent reason i.e. haven't run a tap, used the loo or anything - just sitting quietly and then it suddenly starts. My guess is that is something that starts it from outside the house. All my taps and toilets are fed from the mains and there is a CWS tank in the loft that feeds to the HW and feeds to the CW for the bath. I fitted a check valve on the ball cock in the tank in case the surface of the water was causing the float to vibrate - no change. Any ideas?? Thanks[/b]