Sounds like a loose screw or coin under the passenger seat. It rolls around while taking corners. Sounds like it might be in the seat rail or something because it is certainly on metal.

Anyone else had an issue like this? Any ideas on how to get a look under the seat?

Check the storage box under front of seat...I chased my noise down to a loose pen in the bin of all things....

I had the same thing in my 211- a leatherman would slide side to side and make a "thunk" which I originally thought was a loose seat. If it is not in the bin, then you might have a penny or something sliding around under there...

rattle snake?

Turns out a quarter had gotten into the track on the passenger seat (god only knows how).

Luckily the tech could get it out. I couldn't even see the track.