Last weekend I upgraded the hard drive in my MacBook to a 7200 RPM one and it started vibrating. There's one good thing about the vibration though. My MacBook's Apple logo always lights up when the lid is closed and I unplug the mouse. I've always wondered if that means the hard drive gets powered up when that happens. Now that I can feel the vibration, I now know that the drive is turned on when that happens, so I know not to move the notebook until the breathing light goes on for a few moments.
I read somewhere that screw tightness could affect vibrations. Looks like another disassembly. Or maybe I should wait for the 6 GB upgrade?