The piping is the way it was when we moved in, I upgraded to 3" pvc about 4 years ago when we added a bathroom upstairs but put everything back the way it was. you guys are saying there should be a vent for the tank itself ? as in between the mainline trap and the tank I need to put a line going out and up over the roof to vent the inground septic tank ? Could that be the problem ? the tank cant vent so it is pushing gas back up thru the traps and into the house ? kind of makes sense now that I think of it.