There is a company in Europe that sells a very useful hard drive silencer called "NoVibes".
It's a bracket that mounts in a 5 1/4" drive that holds the hard drive between a few thick elastic bands, and some rather thick rubber pads. Almost totally eliminating any vibrations.
This is very effective with those monster 10K RPM SCSI drives. Also, the 3 1/2" to 5 1/4" conversion provides minor cooling benefits due to the increased airflow. Just cut a few holes in the bay cover, or even add a couple (quiet!) fans.
Edit: Zerotime has been kind enough to bring to my attention a comprehensive review at http://www.dansdata.com/novibes.htm, thanks!