i do iphone repairs and housing replacements. Many if not all 4's and 4S's do it. its the autofocus in the camera. I opened up my iphone5 two nights ago, cause i had the rattling sound when i shake the ip5. there are no other loose parts in there, i isolated it (as expected) to being in the camera housing, more specifically the lens or rather plastic disk thats around the lens in the camera. Its free floating and loose, as it should be. However, some phones have it rattle more than others, as other iphone 5's i've played with only make the sound when tapped, not shaken. It certainly does not effect function of the camera though. So, if you feel that it bothers you, as it does me because i can actually FEEL and HEAR it every time i put my phone down, go ahead and exchange it. If not, keep it, there's no functional issue from my experience. Its up to you.