When the taps (or valves) in either the kitchen or bathroom are turned off there is a strange noise in my converted cellar behind the plasterboard walls that sounds like the mains cold water pipe moving, or knocking, against the plasterboard. I have managed to reduce the noise slightly by turning the mains stop tap a few times, but it is still there. I am aware of the principle of water hammer, and even installed 2 hammer devices on the pipe (one in the kitchen and one near the toilet feed), however it is still there. The noise is not particularly loud, nor does it sound like it is doing any major damage, but it is highly frustrating. I even cored out 2 holes in the plasterboard wall to look at and feel the pipe, however it is not moving and there is very little vibration to the touch. Any ideas ??