My 4s rattled a bit when it was new. I wasn't sure at first if the buttons were responsible for the rattling, or if it was something internal. Then I dropped it a couple of times while spinning it around in my hand to locate the side with the button I wanted to press, and that made it stop rattling for some reason. The front and back glass slabs now slide around a bit, though.
I'm more or less dumbfounded at the popular perception of the iPhone 4 as a "great" industrial design. While it's definitely a great phone, if they don't return to a curved back with the next model or otherwise provide some tactile cues that help the user sense how it's oriented as it's being withdrawn from a pocket, it'll almost be enough to send me over the fence into the Android camp. Failing to move to a bigger screen will certainly do that.
Mine makes a little rattly sound, but it is from the 'bumper' case that it has on it. The contacts on the bumper make that sound for me. When I take the phone out, it is silent when I drop it from a half inch or so (other than the clunk sound the phone itself makes).
Dropping the 'bumper' by itself also induces the rattle sound.
Other than that, I have not heard of any iphone4S rattle sounds. Does yours have a 'bumper' case on it?
I can't believe that slipped though. It's a blot on the impression of the iPhone 4S as a solid thing.