I recently had my iPhone 3GS replaced at the Genius Bar. On the new phone, I noticed that it would vibrate really loudly, as if something in the case was vibrating along with the vibration motor. Here's how I fixed it: 1. I unscrewed the two screws on the bottom of the phone. 2. I took a suction cup and lifted up the display to about a 30º angle. 3. I moved the display over so that I could see the vibration motor (with it still connected). 4. I took my Philips #000 screwdriver and unscrewed the screws that hold the vibration motor in. 5. I then put the screws back and tightened them pretty tight. 6. I tested the vibrate function, and indeed it was much quieter, like my original iPhone!