I had this same thing, kept noticing it when I was on a call and kept hearing this little clicking type noise....Then kept hearing the same thing when I was tapping the screen while surfing the phone. This all was happening about 25 days into having the phone, called ATT, went to a local store and they replaced it no problem.....While at the store the guys shook the new phone and nothing....the next day, the same rattle is back! Not sure what to do at this point, had a Gs2 and it rattled also a little, but for sure not as much....Check my wifes new iphone, it surprisingly has a rattle to!...but man are iphone so much more solid, these Sammys lose so many points on build and feel....anyway, I did notice on all the phones I checked that when you pinch the area around the speaking you put your ear to and shake the phone it kind of stops the rattle, so maybe this is something to do with the speaker?
I dunno, it's pretty disconcerting considering it could get worse over the first year and of course magically break 1 month over the year warranty .......
Anyone have any other ideas on what to do here??? I have a manufacturing background so I can understand what Sammy might be considering if they know about a build quality issue to 10 million units or how ever many they sold, but I'm wondering how it can officially be brought to their attention.???