If you want to spend some money, you could invest in a fan controller. I just got my Vantec NXP-305 today, and it's working really well so far...I have a Vantec Tornado by the way, so that thing is LOUD

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The controller will allow you to get the full RPM of the fan if needed, but also allow you to turn it down when you don't need or want it to run at full speed.
You get a pair of cathode lights with the NXP-305 as well, so that's an added bonus.

They look pretty good too.