There can be alot of reasons a pipe vibrates. One has to do with toilets flushing. Check out the answer earlier in the messages about Machine gun noise in piping, for the complete answer. But many times toilet valves have diaphrams in the ballcock (fill valve) inside the tank. These diaphrams are made out of rubber; when the rubber (neopreme) gets old, the gasket looses its elasticity. Once the elasticity is gone, it can no longer mold against the seat inside the fill valve and shut off the water. When this happens, it shuts off rapidly and turns back on rapidly. This creates a harmonic resonance in the piping that groans or growls. The shower valves can have a gasket on the inside that loosenes. This begins flapping and creates the same noises.
The piping can also cause noises from everything from too high pressure, to movement from expansion and contraction.