Here's a tip:
Don't drop your Galaxy Nexus. I did, from 5 foot onto solid concrete. Some how, I have no idea how it was externally completely perfect. Not a scratch on the screen or casing.
But now every time the thing vibrates it sounds like the screen or something else is vibrating. I opened it up and took a look and I can't see any physical issues or something loose. So I can only assume the vibration motor is a little knackered. Ah well. Just glad it didn't completely smash.
Mine does this too after I dropped it.