I went to a number of stores and held as many MacBooks as I could find and they all seem to vibrate some, regardless of hard drive model ( FYI - the Fujitsu seems to be the most common model). I believe it to be the fan off balance.
My particular MacBook just seemd to get a lot worse after it had been on for a while, so they replaced it at the Apple store where I bought it. It would shake so hard after a while that the screen would shake.