This has happened to me before, and it sounds like your vent pipe is clogged. The little birdies actually built a nest inside my vent cap. Somehow it got cocked at an angle and was just enough space for our feathered friends to stuff it with dead grass and mud. This may not be the case for you, but that kind of thing does happen. Wasps and hornets will also find it to be a great place to start a hive. I would have a look there.
"Walk softly...and carry a lot of tools!
2016 Ford F250 Super Duty Crew Cab, 4x4
2021 Palomino Puma 27RLSS