As much noice comes from vibrations...
Did you know you get almost as much noise from vibrations of the fan rattling inside your case as you do from the movement of the fan blades and air?? An easy fix for this is to use Neoprene lined washers you can find at any home depot. They made a huge difference in residual noise in my system.
You can find them in the washer aisle. The are called Neoprene Arandelas, Bonded Neoprene Washer. The code on the box of 100, which is only $2 is 3/8OD - #8 Scr, and the stock number is H#35018. Use 8 for each fan with full 2 or 2.5 inch #8 screws instead of the tiny fan screws, they really kind of stink. Use a screw that goes all the way through both sides of the fan, not just one. And use all four screwholes, not just cheat and use the 2 diagonal from each other, cut down on the vibrations of the fan, and you will cut back on noise. Not much, but, defintely enough to NOTICE a difference for the $5 you will spend on real screws and washers...
Be COOL! :grinthumb
Kris