Update : I've driven around for the first time with the new unit in the car. Damn : it's loose enough to rattle. And it does rattle. Now, I've checked this out with my previous mk2a and there was a difference : the 10 gig is "steady", in it's own sled AND in the sled of the 60gig. The 60gig sits "loose" in both sleds.
I'll probably open both units now and switch HD trays. That should solve it.