simple-water hammer,when you use hot water the header tank is replenished via a ballvalve,this is either sticking or creating waves in the tank, therefore the valve bounces and starts vibration through the cold main,1-check all pipes are secure (1clip per mtr) 2- replace ballvalve-3- fit anti hammer device available at most good merchants.
To prove it is water hammer-when the noise begins turn on a cold tap-the noise should stop or reduce(in effect you have reduced the water pressure flowing to the ballvalve)
alan peck, January 2009
