I thought about that... give it a try and let us know.
Since so many of us are complaining about the same types of noises, we can divide and conquer to try and track 'em down! I'm going to try securing the seat belt latches - and maybe the housings, if I can come up with an idea how - and I'll report back.

I got my son to sit in the luggage area and went around the block. He said it seemed to come from the top of the rear window. I suspect its something to do with the wind deflector. Not one other bump or rattle in the car except this one :-|

That definitely sounds like something loose in the roof spoiler... might want to take it off and have a quick look - it's probably something pretty obvious.

...

No luck with my test on the seatbelt latches... I secured them in place well above the little catch on the strap, and I still heard the rattle a now and again. I'm still almost positive that it's coming from either the center seatbelt recepticle on top of the seat, or possibly from some sort of vibration between from the gap between the split fold-down rear seats (?)

Romeo, any luck with your test on folding down the seats?
Anybody else...?

My only rattles come from my kids and the pano-roof. For the kids I just crank up some SOAD and on the pano I have to open the shade an inch or two and then shut it. That lasts about a day.

I also noticed that the rear sunshades rattle if they arent lowered correctly. Whatever that means!? I can raise them and let them seat properly and they stop rattling.

ROOT CAUSE DETERMINED... I think
 

So far, every time I think I nailed down the cause, I proved to be wrong after another day or two of driving... but this time I really think I have it!
The other day I noticed the vibration noise when I shut the driver's side door - a golden opportunity to try to track it down, since I could replicate it without having to drive around and aim for rough roads. :tsk:
So after some repeated door closing, I verified for sure that the noise was coming from the center of the rear seat, as suspected. So I climbed in back and tapped my fist around on top of the center of the rear seat until I heard the exact same noise that's been bugging me for weeks. It was clearly coming from the gap in between the seats... but as anyone who's already checked this area can attest, there's nothing in there that should be making this kind of noise!! :dunno:
Well that's just it! Even though the noise absolutely sounds like a plastic-on-plastic (or maybe plastic-on-metal) vibration, that not what it is... it's actually the sound of the two leather seats rubbing together as the seats vibrate slightly. I'd have never believed that they could make the sound I was hearing until I watched it happen 6 inches in front of my own two eyes!
With one hand tapping on top of the seat, producing the noise, I could stick a finger into the gap between the seats and it would go away entirely... it's as though the way the seats come together, they form some sort of acoustic resonance chamber that picks up some slight vibration and magnifies it off the leather pieces rubbing together... or whatever :confused:
(This, by the way, explains how lowering the armrest usually makes it go away completely, as when the armrest is up, it spreads the leather opening out a bit and effectively squeezes the seats a hair closer together.)

So now the next step is to attempt to verify my theory by driving around for a few days with a towel or something stuck in-between the seats, hopefully in blissful silence :D
for those of you with the same noise, give this a try and let me know if it works for you too.
Good luck!

Had the same problem with the seatbelt and if your noise is the same as mine, buckle the seatbelt, it should take care of it. Hope that helps!

my rear drivers side seat belt flaps in the wind and slaps against the seat; which, in turn drives me nuts!

Off topic...where did you get that picture in your signiture? Do you have a high resolution version?

sorry it took so long to reply. i've been trying to find it again.
i think i found it here. i do not know if there is a high-res image. i'm still searching.

oooooooooooooo, u know what? it DOES sound like styrofoam rubbing against each other, now that you mention it. gotta check that tonight.