Your PVC waste pipes are behaving correctly, and noisily. They are tightly secured to the wall framing. When they expand from hot water (get longer) or contract and cool off (get shorter) they slide along the wood framing they are secured to. The temperature change from hot water draining through them causes a rapid expansion of the plastic pipe, and a plastic to wood framing “earthquake”. This earthquake is a “bump” noise or ticking sound as the pipe gets longer with the warmer water flowing through it. When the hot water stops draining through the pipe, then the pipe gets shorter again as it returns to a neutral temperature and size.

When the house was built the plumber tightly secured these waste pipes, and they jump or rapidly slide against the wood framing … which makes the sound. I call it an “earthquake” because it happens in an instant and not gradually. This instant move causes the bumping sounds which happen both when the warmer water begins to drain and again as the pipes cool off.

The solution is to cut open your drywall and loosen all the clamps inside the wall so the pipe slides smoothly along the wood framing, then close up the drywall, finish the drywall and paint. This is not usually not worth the effort because the pipes are not causing any damage, only sounds.

Consider the wall noise as the personality of the house and live with it, or do open wall surgery to stop the noise. It is highly unlikely that damage is part of this occurrence.