Hold the bottom of the phone with two fingers, along the side bands. Tap the rear of the phone on or above the Apple logo with a finger on your other hand. With my 4S it sounds like something is loose. Thought maybe it was one of the 4 buttons (sleep, vols, silent) but I put pressure on those while tapping and same sound. Then bought maybe it was the internal vibe motor. I started the motor while tapping, same sound when it was running. Any ideas? Does your 4 or 4S do this?