Gravity Furnace Thumping Noise

Hello, My first post on here. I can't seem to find the right keyword to find anyone else with this type of issue. Simply put, occasionaly my gas furnace will ignite and make the following noise. It's as easy as turning down the thermostat to kill the flame, then turning it back up to re-ignite to get rid of the noise. See video https://www.youtube.com/watch?v=_I0k9P_-dqM&list=UUoAo324d8kMUsGs-TCFGDNA Furnace is a gravity furnace. Suburban WD50-31 (I think) No clue on age, assuming 15+ I don't see any way of controlling gas input. Air input adjusters don't seem to make a difference (nor do they look like they would work...)