One thing I'd suggest is to try booting up your PC with the case off (just power on) with one item at a time disconnected (except of course for your power supply, that's a hard one to disconnect). This way you can find the noisiest item(s). I found that one of my older 10 GB drives was the noisiest item by far. I managed to quiet my PC significantly by setting the disks to power down after 15 minutes. Regardless of what you find, you'll know for sure what your #1 target is.

-Zeke