Seigfried from Richmond Hill, Ontario hears some very strange creaking whenever he pulls the plug and lets hot water down his drainpipe. He sent us an audio recording of the noise which did help me to identify what was going on.
Expansion and contraction of plumbing pipes, because of a sudden change of temperature, can make strange noises when it is restrained somewhere. Either it is rubbing against the structure of the house, or there is a plumbing strap that is too tight. You need to follow the pipe (not easy when it is in a wall) maybe using a stethoscope (from the kid's toy box?) to locate where it is restrained. Then loosen the clamp or get some rubber padding between the pipe and the structure it is rubbing on.