My parent's bedroom at downstai and there is drain pipe inside the drywall; along with middle of closet, when flush toliet at upstair, the water run through the drain pipe makes lots noise, especailly night time, anyway I can put something inside the drywall to reduce the noise from the pipe? any suggesttion.
You apparently have PVC drains, and they are usually noisey. You can insulate them after remving the drywall in that stud bay. It should help, but it may not be good enough. A better solution, but expensive, is to replace the PVC with cast iron.