While idly tapping the back of my iPhone 4S I noticed a rattle sound the closer I tapped to the camera lens (or lense? I never know!). It's mentioned in other forums that it might relate to the camera lens or the accelerometer but those cases complained of it rattling when shook, my rattle only happens when the back is tapped. The rest of the back produces a solid noise when tapped. Worth taking back or complete OCD?