Glad you were able to resolve it. I never thought about it being the vent. I did have a drainage issue one time and the advice was to flush hot water down the sewer vent on the roof. It worked that time for some reason but the real problem was that the sewer line was old as hell and in bad shape so when the ground was soft again we had to replace it.
The sewer line was my first thought too, but I'd scoped it recently and it was clear. I've never had to mess with sewer vents beyond "I know what they are and they just work" so I'm glad Poal was able to point me in the correct direction.
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