Outside of music, I prefer my working environment to be as quite as possible. One thing I’ve found which helps is a hard drive enclosure called “Smart Drive 2002″.
The enclosure itself fits inside a 5 1/4″ drive bay and houses a 3-1/2″ drive. It is basically one big heat sink that completely encloses a hard drive with a combination of dampening foam and metal contact to reduce noise emissions as much as possible.
The hard drive inside will run a little hotter, but as long as the ambient temperature isn’t too high and your machine has a base level of air flow this should not be an issue. (I’ve run a 150GB and a 120GB in a pair of enclosures stacked on top of each other for a year now and have had no issue with overheating.)
The enclosure comes in a standard version and a copper version, the latter providing more efficient heat transfer, marketed as being suitable for the hottest drives and possibly even lowering their operating temperature.