I've had an ongoing problem with my basement toilet. Every time it rains or we have high humidity I get a strong sewage smell coming from inside my toilet bowl. There is no smell coming from the shower drain or sink next to the toilet. The septic tank has been pumped and line checked. I got up on the roof and ran a PVC pipe down the vent pipe in case any blockage. Any ideas as to what's happening here and how to fix??