One can also baffle the inside of the computer case with the speaker batting available (at least it used to be) at Radio Shack. One must be careful not to impede air flow, but when I had four of the original Barracuda 7200 RPM drives (ST32550) in a single machine, a layer of that stuff made a lot of difference in the noise level outside the machine.that foamy stuff is exactly how I quieted the 15,000 RPM hard drive in my computer at work. I love that idea.
It's not much fun to work with though, as it's either a lot like, or exactly like but a different color than attic insulation.