My house was built in 1949 and we have a septic system, gravity fed. When the pump comes on which pumps the crap to the septic tank the toilet flushes, it is below the toilet in the basement and they share the same vent. My feeling is that it is a clogged vent pipe so I got on the roof and tried vacuuming it out with my shop-vac by shoving the tube down as far as I could (about 6 feet) and then took some fish tape and running it down to try to loosen anything while sucking. I did get some stuff out because I have a lot of pine trees and the vent stack is a 2 1/2 incher so it could collect debris. I have since covered it with a wire mesh but I still have the problem. Does anyone know of a situation like this that could impart some wisdom? I hate the smell when the toilet empties like that! Thanks very much.