A couple of months ago we had a new boiler (regular) fitted. A couple of weeks later my wife said there was a strange noise coming from the spare bedroom en suite, which only occurred when I was using the basin in the bathroom. I assumed it was something to to with the HW/CH pump. The next time it happened I checked and was most surprised to find it was the mains cold water pipe which fed the instant-heat shower that was vibrating. I couldn't understand how the pump could be responsible, as it was nothing to do with cold water and at least 10m from the en-suite pipe. It wasn't, of course, and a bit of internet searching led me to the float valve on the cold water tank; that was indeed the cause.
So yesterday I removed and cleaned it, but it made no difference; when the tank filled and the float valve started to cut off the supply, the vibration started again. Today I replaced the old float valve with a new Pegler float valve. And the vibration is even worse - now we have a foghorn! One thing I noticed was that it was possible to cut down the sound by flexing the side of the plastic CW tank by pushing on the valve inlet. When I removed the old valve I noticed that the reinforcing metal plate was mounted horizontally, and that's the position I replaced it in. But it seems to me the tank would be stiffer with the plate mounted vertically, so that's what I'm trying. At the moment things seem to be better, but they may not last.
If the noise returns, I'll try partially closing the tank isolator valve first to see if that helps, as that's the easiest thing to do. If it doesn't, I'll try the "yoghurt pot" dampener on the float arm.
But if all those fail, I think the easiest thing is to fit a Torbeck valve instead of the standard float valve.