I currently have a on contract ATT 4S which I am returning as I just purchased the off contract unlocked. Sure enough the off contract rattles when its vibrating which is annoying. If I press lightly on the back plate while vibrating the vibrations stops. The problem is there is a very thin tolerance between the back plate and battery for some of these phones coming out of Foxcon. I have seen these types of issues with many phones such as Blackberrys and the fix is to try another back cover or put a piece of black electrical tape over the battery so there is no space to cause the rattle. I am not going to Exchange it because the phone is perfect otherwise. No yellow screen , bad pixels etc not to mention I put my last power support screen protector on there perfectly. Does anyone know where you can get the special screwdriver to take the back off? I am going to try switching backs with the other iPhone 4S before I return it. If that does not work I will place a piece of tape over the battery which I am sure will fix it. I may also see if the Apple store will try another back or look at it. If this is not something you are comfy with then I would take it to the store. They will prob swap it out. FYI the other rattle is the camera AF element and is normal.