We live in a 7 year old house. 2 years ago we began smelling a septic odor in the basement on the septic side of the house. We’ve had the system checked, and it’s okay. The septic service man who pumped our tank recently suggested extending the roof vent pipe. Why would this help. Do we have a leak? How do we test for one? The smell comes and goes with no apparent rhyme or reason? Please help!