I've changed my feeble mind! I don't agree new rad, hope you haven't bought one! Didn't read what you said carefully enough. The problem occurs when you draw hot water. Therefore you look for which pipes get hot then to find where the problem is. You could cut out some copper pipe and replace with plastic, to take the stresses, but you have to watch how you do this, generally not within the first metre of pipe from the boiler and whatever else the boiler maker says. Also, the copper pipes usually provide earthing electrically. If so in this case, you mustn't compromise this (fix earthing clamps to the copper pipe and run earth cable along the plastic pipe run to the next bit of copper). If the new plastic pipe includes a 90 degree bend, then the isolation is most effective. As Slippy said,' noise travels'.