I did some experiments with a new AL PB a little while ago, and here's the trick:
If it only feels like it's vibrating when you actually move your hand across the surface, it's your *hand* vibrating, not the PowerBook.
The thing is, the just-slightly-bumpy texture of the surface causes your hand to sort of skip across it (a microscopic version of the bump-bump-bump effect of draggin your hand across rubber), which ends up feeling like vibration. I'm nearly certain this is what it is, as when I did the experiment it was the same even if the PowerBook was off, unplugged, and with no battery, so it *couldn't* have been electrical or mechanical.
Try it and see if this is what's going on for you. Wigged me out at first, too.