the EASIEST thing to do (or at least try) is to go to Advance Discount Auto Parts and pick up about 5' of small air hose. Use an Xacto knife and cut it straight down the middle. Use this as molding for the fans, it will reduce any vibrations, and probably cost you around 89 cents a foot. Also, try using grommets in the fan screw holes. This may help..... Xoxide.com also has as Quiet PC section...
http://www.xoxide.com/quietpc.html this may help.