Cold water feed makes a right racket

Hi can anyone help me please. We have a back boiler for our central heating/hot water. When we use a hot water tap and turn it off there is a right rackety noise which goes on for about 30 seconds. When I looked in the airing cupboard, which is where the hot water tank is housed it seems to be the cold water feed pipe that vibrates but the noise is coming from where it seems to feed the hot water tank. I have tried opening and closing the stopcock which works sometimes but when a hot water tap is used the noise comes back. Anyone got any idea's ??