An update on this stuff.
I did a bunch of testing at a friends house who has oodles of old HDs. After testing over 10+ drives I found that Seagate "Medalist" drives were the best (clocking in at 2gb on two platters).
The way they are built makes it easy to access the voice coil. The click is satisfying/clear, and once you remove the little rubber bumper that helps preventing the clack, it's even louder.
I have two identical drives now, and each has it's own unique sound, which is interesting.
Here is a pic of the testing:
(the seagate is the bottom/left most drive)
Now as to driving them. We started by just sticking the speaker out of a home hi-fi setup and it worked right off the bat (once we found the points to connect to). Music sounds like music but very quiet/tiny sounding (like a DIY turntable with just a needle going into a cup).
We setup a puredata patch to spit out low frequency squarewaves that changed pitch and rate of change randomly.
This worked the best as it threw the needle around pretty nicely.
At the right volume level (as in, hi-fi volume crankedness) you get the most awesome clickity-clack going.
The problem (at the moment) is that it has to be very loud. Like plugging the speaker back in is "we're have a house party and our neighbors asked us to turn it down" kind of loud. His hi-fi is rated at 40w (or was it 60?), point is, pretty high, and out of reasonable battery powered volume.
I tested it just now, for fun, with my 386-based pocket amp and it made not even a blip.
I really want to build this into a self-standing wooden enclosure, so I want to avoid having a massive power amp in there, then of course is the problem of producing the actual audio. Since squarewaves aren't too weird I figure an Arduino can spit that out and it's low power consumption.