Does anyone have a suggestion on how to make my hard drives quieter? Is that even possible without buying new drives?

Yes, you can try this. It supposedly works very well, effectively silencing your HD with +/- 10dB. (which is about 3x less noisy). Just be very sure to get a Smart Drive, and NOT a Silent Drive. The Silent Drive is about the same thing, except that that thing doesn't dissapate the heat effectively with 7200RPM drives, and thus "cooks" the HD.
The Smart Drive however, is very good at keeping the HD silent AND cool, even with 7200 RPM drives!
Review here. (this is a review of the first generation Smart Drive, that's no longer for sale. It's successor, the Smart Drive 2002 is even better)