When our second-floor shower is used, we occasionally get a sewage smell seeping out of the sheet-rocked pipe chase in the floor below. This leads me to speculate that the PVC drain pipe is cracked or has a bad joint. It is a finished wall so I dont want to tear into it, if possible. The house is about 10 years old. Could it be a faulty vent or other problem? Do i have any hope of repairing this in a non-destructive way? Any advice will be much appreciated. Thanks!