If I may, I recently purchased a Panasonic DMC-TS3 — a ruggedised camera. Since I was downgrading from an SLR, I took the little thing with me everywhere in my pocket. Needless to say, I inevitably started to notice a rattle coming from it! It was troubling, as the thing is meant to be tough, right? So in anecdotal use, I began to notice that it only rattled while off or in Playback mode. Curiouser and curiouser.
I always try to see the good in people and the right design from companies, so I hope the rattle stems from the following:
Eventually stumped, I went online to inquire what the rattle was about. As a rugged camera, it was drop tested to have no defects at 2m and lenses undamaged from much greater heights… lens undamaged? Turns out to protect the lens elements from drops and shocks, the lens group is released and floats around within a cushioned compartment in the camera (though it certainly doesn't sound too cushy to me). The principle being that accelerations the frame is subject to do not translate directly to the lenses and risk cracking them. I know the iPhone isn't particularly rugged a far as drops, but something we put in our pockets every day can be subject to a lot of stresses.
I really hope that's the reason why my brand new and first iPhone is rattling, eh Apple?… Apple?