Does it happen in the mornings, after it has been cool/cold at night?
PVC drain pipes would be cold, water draining (drain pipe runs from upstairs, down through a wall) from a shower would be warm. The PVC drain pipe will expand and make a sort of popping sound against the metal clamps that are holding it in place. This will usually stop in the summer, but then re-appear in a year.
This is the theory that I settled on, but now I have to investigate the weasel idea.