I pulled the end off the hopper assembly this morning to find that
#30 Plate, Fan Air Diverter was hanging on the
#29 Inner Fan by one
very loose 6-32 machine screw. The other machine screw/nut was rolling around in the bottom of the hopper assembly.
A little background... I picked up this YS640 at ATBBQ yesterday and drove home to northwest Arkansas - so I suppose it could have come loose during transport. With that said, I bought a comp cart model... so I hope subsequent trips end with better results.
During re-assembly, at least two of the self drilling screws wouldn't retighten - and I was being gentle with them (Klein nut driver). It has been my experience that those things just aren't made to take in/out. Hopefully this is the last time I have to get inside the hopper assembly.
Anyway, everything sounds great now:
https://youtu.be/qgn5t0I_NBs