I've got a slight rattle in the air vent in the center of the dash. If I press down on the vent cover (the triangular one on top of the dash) while the rattle is happening, it stops.
It's also possible for me to reproduce the rattle by banging on the dash, just to the right of the vent - you can hear some small plastic piece rattling against another.
I'll have the dealer look at it on the first service - it's not bad enough to make a special trip now.
Hope this helps, maybe you've got the same thing.