I have a two story house, and whenever someone flushes the toilet upstairs its sounds like a waterfall inside the wall downstairs. I don't see any place where water is damaging the wall so I'm just curious if this is a normal sound or if I have a problem.
This is a common complaint with plastic drainage piping. I've not heard of anything that helps except to replace with cast iron. People have tried wrapping the plastic with insulation, blowing in insulation and installing 45's to break the fall. All leave a sound. I'm not sure about installing lead sheets under the wallboard but some have good reports.