Since you don't have anything in the optical drive (which can vibrate like crazy, espeically with a slightly unbalanced CD in it), that sounds VERY not normal to me. Even desktop hard drives have little if any discernable vibration, and I've never seen a laptop drive that you could even tell was running. It's probalby dying, and I'd get it checked before you get anything significant put on the drive. You can always launch Disk Utility and see what the SMART status is, but that'll only report a problem if the drive itself thinks it's dying, which it may not realize (yet, anyway).
That said, sure sounds like a "feature, not a bug" to me. Have fun with your Mac while you have fun with your Mac! Add in integrated remote control via iChat, and you've got the ultimate long distance relationship helper. Apple could sell millions.
There was an obscure PS2 attachment for a particular game that had the same effect, if memory serves.
(On an unrelated side note: Between this and the "I lost my virginity" post, what's with all the eyebrow-raising thread titles recently? I admit, it does get a lot more views than "I think I have a hard drive problem," though perhaps a much higher signal-to-noise ratio.)