Knocking Water Pipe in Shower

When the shower is turned on (hot or cold), the pipes knock (loudly) twelve to fourteen times– then it stops after the water has been running awhile. It is PVC pipes. My plumber claims that the pipes are hitting something and the contractor has to fix it. My contractor stated that it is just natural for new plastic PVC pipes to make noises and that it is just contraction of the pipes. The knocking is obviously coming from the pipes in the wall which are accessible from the other side of the shower. Any suggestions– who is right.