We live in an old farm house with one vent pipe and the toilet bubbles when using the shower or bath. The septic tank was just drained a couple of years ago so I don't think that's it. I think it might be the vent pipe because the toilet wants to overflow if the tub is draining but otherwise it flushes fine. Could it be a blockage in the vent pipe? If so what is the best way to clean it?