I tried rubber washers and gaskets, and found that the thing that helped the most was making sure the fan screws were all equally tight, and secure (but not too tight that it warps the fan housing in any way).
Also, if you are running high-speed fans, you might be better off investing in a fan controller that will allow you to undervolt them -- some fan controllers are also PWM enabled, so that they ramp up based on load, like your CPU fan does.
And some fans are just plain loud, especially sleeve bearing fans. What kind of fans do you have and how/where are they mounted?