How to fix iPhone 5 battery rattle inside
For those who have a battery rattle iPhone 5, you can try to fix this problem by running the vibrator for a few minutes. There 2 ways to do this, download and run free vibrator app in AppStore or turn your iPhone 5 in vibration mode and use another phone to call your iPhone 5. While the vibrator motor is running, leave your iPhone 5 on surface (table top). This will help the battery vibrates and sticks to the adhesive on the housing.
I accidentally fixed my iPhone 5 that way. If the above way doesn't fix then bring the phone to Apple store. Good luck and let us know if it help.
iPhone 5, iOS 6