PVC Drain Pipe Knocking

My wife and I recently purchased a 2 story house. Within the first couple of days we noticed that when the faucets or showers upstairs run that there is a hard knocking (like somebody knocking on a door) on the wall that houses the main PVC drain pipe.

From what I have read, it sounds like the PVC expands and can rub and make noise. What can be done about this (other than replacing with cast iron pipes)? Or are there any other factors that could cause this noise?