Not all FDB drives are quiet. My maxtor 80gb was the first FDB drive they developed, they claimed it was quiet, but its noisy like a mother fcuker (in actuality it was giving my case lots of vibes, but even using rubber washers to pad it didn't do any help, only taking it out of the case and on the floor did the trick). Seagate FDB drives are dead silent. And from what I hear from hot maxtor hdd deals, those newer FDB 8mb cache drives are not that quiet either. Many people say their maxtor or western digital harddrives are quiet, but maybe their fans are too loud that they drown the hdd noise. Also, I tired out those smaller diamondmax 8 or 9 drives (the maxtor ones with single platters up to 40gb or so only), and those are pretty darn quiet to nearly silent.
Heres what I got:
Western Digital 40gb 7.2k rpm 2mb(ball-bearing) - loud on idle, makes louder noises when reading/writing
Maxtor 80GB 7.2k rpm 2mb (fluid) - no noise on idle, makes loud noises when reading/writing/turning off (when in pc case) - no noise on idle, very very faint noises when reading/writing (when on floor outside)
Seagate 40gb 7.2k rpm 2mb (fluid supposedly) - always silent, can't be heard unless you have no fans or other noise making component of any sort, and if this is the case, can only be heard at most 1 foot away
IBM "Deathstar" 15gb 7.2k rpm 2mb (ball supposedly) - if i can recall pretty darn loud like the WD drive