I just recently purchased the Seagate Cheetah X15 18.4 gig, w/8 meg cache (ST318452LW), together w/an Adaptec 29160 controller, and it's absolutely quiet.
I presently have my case open, and I don't hear anything, except for my cooling fans of course, which aren't that loud. The HD isn't isolated w/any form of rubber. I'm quit happy w/the drive, which by the way is my first SCSI HD. My previous setup was a four IDE drive RAID 0 setup.
While the new WD 100 and 120 gig w/8 meg cache HD's may be fast, in relation to IDE performance, but I didn't need all that HD space. Besides, I I wanted "the" fastest HD available.
As an engineer, my system at work has only around 10 gig HD (IDE) space, and to date, is only perhaps half full w/large solid model CAD files (Solid Works 2001+), after over two years there. Some of these files are quit large and take over 5 minutes to load. Boy I wish I had an X15 there.
Anyway, rather than continuing to increase the space on HD's, I wish they would start to concentrate on performance and larger cache.