We had an issue with ours while running the fan in the bathroom. It was more pronounced as the tank filled. Ours was (is) under warranty and eventually our dealer went on the roof and discovered that the tank vent pipe was cut too short, ending a couple of inches below the roof line. They reportedly installed a coupler and sufficient pipe to bring it up where it belongs and we’ve had no further problems.
Current: 2020 Rockwood MiniLite 2205S
Past: 2018 Flagstaff MicroLite 21FBRS
Past: 2005 Flagstaff Classic 625D
TV: 2016 Ford F-150 3.5 EcoBoost