Think I figured it out. Seems it was attached to bottom metal rail of window, maybe as a spacer or something?? (Found some of the green, rubbery substance had rubbed off on inside bottom of door). Appears to have been spot-welded or riveted (vs screwed), so reattaching aint an option...
Window works fine, so I suppose I'll trash the part.
Probably the source of my "rattle-thunk" too.
Onward.

Ragtopper