Buffetting in furnace

Hi All - I have a Coleman TM9V080C16MP12CB natural gas two stage furnace. I’m having an intermittent issue, usually only when it’s running on the high stage. There is buffetting/air pressure change in the furnace that’s making a very audible noise. If I crack the furnace door you can see it bounce open and closed as the pressure is changing (see video). There is also a very faint smell of gas, as if some gas is being pulled out of the burner before ignition. If I totally open the furnace door, it sounds fine. So it seems like the issue is only happening when it’s drawing in air from the outside.

I checked the air intake and it’s clear. I also put a shop vac on it and it pulls air through fine. And again, it only seems to be intermittent.

Any ideas? Thanks!