This is correct. The new pipes were of a different diameter (problem #1) so there was a water hammer, a pressure wave moving back through the pipe when the tap

This is correct. The new pipes were of a different diameter (problem #1) so there was a water hammer, a pressure wave moving back through the pipe when the tap was turned off. Being plastic pipes, they moved quite a bit when this happened. The pipes were also installed by non-certified plumbers (problem #2), I'm pretty sure. In many cases, the pipes were not tied into anything in the wall. We had to fix the last couple of these issues ourselves, where the pipe was actually resting against the drywall, and would move, thumping against it. All the while the contractor is saying this is normal and expected. My wife, a certified Professional Engineer, something in her finally snapped. She said "get in the car, we're going to Rona[0]". I dutifully followed her around with a basket that she filled with equipment and supplies. We cracked open the walls, tied in the pipes correctly, patched the holes, repainted. Home back to normal now. [0]https://en.wikipedia.org/wiki/Rona,_Inc https://en.wikipedia.org/wiki/Rona,_Inc.