I was wondering about the fans also!
This has to be the strangest thing I’ve seen yet concerning the F6B.
Very interesting. As I recall, there are no points on the engine where any rotating assembly extends outside the engine (i.e. no pulleys, harmonic balancers, etc). Just to be sure, I found this ebay listing that has nice photos of a GL1800 engine (which also happens to be from an F6b).
https://www.ebay.com/i/254364995116?...saAvNyEALw_wcB
The engine in these bikes is a stressed member (i.e. the frame bolts to it...it's structural). Has the bike has suffered an impact or wreck?
Just a crazy idea: put the front tire up against a curb or something that won't move and bounce it without touching the brakes.
The reason I was asking about wrecks is that with an engine that's a stressed member, it's possible for a wreck to damage an engine mount and chattering noise when the frame/mount flex as you apply force by bouncing it. To be fair, I'm grasping at straws. Based on the video, though, it doesn't SOUND metallic.
I agree with SeaSteve and billsim...probably time to start removing tupperware to see if it goes away or at least to get a better look/listen at it.