martyh

RobM ..as Osem suggested it is most likely expansion ,one of the most common noises come from the contact point of the rad and the wall bracket .As the rad expands it moves on the bracket and can create a randome metalic noise as you are hearing .There are a few solutions ,one is to try and get the plastic bracket clips supplied with new brackets and as far as i know not sold seperately the other is to lift the rad off the brackets and rap some electricians tape around the strap on the back of the rad this will take up any slack and stop metal to metal contact

hope this helps