612 rattle

My '06 612 F1 develops an intermittent noise at speeds above 60mph or so. It's a sort of rattle that sounds like a slapping noise. It's on the passenger side, but hard to tell if it's in the door or behind the glovebox. The underfloor panels all are all installed correctly and it's not the wing mirror or wiper as far as I can make out. Grateful for any ideas.
Thanks