Just to add my tuppence, I had an annoying rattle on the passenger side, assumed it was the window channel or something.
It would only ever do it at motorway speeds, so couldn't check it on the driveway, although I had the OH pushing and tweaking to see if it would stop with a bang in a certain place.
I recently replaced both mirror rubber gaskets as they'd started to go on the corners, and found a loose speaker securing screw behind the speaker cover. Once replaced and tightened, problem is solved.
A similar one on the Spyder was a rattle behind the headrest. I assumed it was something in the roof mechanism, so didn't even try to solve it. Turns out, it was a screw that holds on the plastic/leather cover on the front part of the roof cover. Tightened and job done.
Basically, if you've got a rattle, don't expect the worst! It appears although they put a bit of blue thread lock on the screws, it's a bit ineffective. Looking for the obvious (loose door trim screws/speaker screws/etc.) will solve a large majority of annoying rattles!