I've had mine since November, generally use it in the horizontal position, and haven't had problems with it rattling (and I've got a car with a not-very-smooth-ride). That would suggest that the mechanism is not always loose. I am nervous about it wearing loose over time, because it is surprisingly easy to move by hand, and seems kind of delicate.
tstreete, nice to see you are still checking in...I remember you were one of the first to get a car kit. In the landscape view, my unit does not rattle, but in the vertical position it rattles a lot and I often keep it like this to charge the phone or listen to music, etc when I'm not using the nav app. I'm going to exchange it and see how I fair while waiting to see what the word on the Magellan kit is. Thanks for your help.