Ideally, the best bet would be to use a more massive pipe. The less massive the pipe, the more it will move in response to water flow through it, and that means the more noise it will make.
Is it possible to replace that plastic pipe with a cast iron pipe?
If that's not feasible, then phone around to the HVAC contractors and find out who sells pipe insulation in town. Use pipe hangers to hold the pipe steady in various places and install pipe insulation between the pipe hangers.
Last edited by Nestor_Kelebay; 08-08-2010 at 05:48 PM.