I have found that carefully-chosen 10k and 15k SCSI drives are actually quite quiet and cool-running. I've had especially good luck with the Cheetah drives with fluid-dynamic bearings, and some of the later Hitachi UltraStar drives. I had two 72GB UltraStar 10K300 drives in RAID 1 in a quiet workstation... could hardly tell it was on, with the usual mods to reduce drive noise.

Remember that the same bearing and motor upgrades that make desktop drives quiet also make server drives have longer MTBF. Cooler-running is also a reliability improvement. Not all server drives are quiet, but they're not all screamers either.