When I use the 2nd floor drains in the bath or sink, I can hear a drip in the wall. It starts out relatively fast; one drop every second or so, then slows to a drop every 3-5 seconds. I think all the drain pipes are PVC. There is no visible water damage on the wall where the main waste comes from the second floor, then flows out to the septic tank. Any advice on how to locate (and fix) the drip without tearing open the wall would be greatly appreciated. Thanks!