If you have alot of fans (more than 2), you could use a fan controller/Rheobus to control the voltage's of the fans. The less power going to the fans means they run slower so less noise. if you increase the voltages, they rotate faster giving more cooling.
You could also try noise reduction padding to line the sides of the case to the noise is absorbed.
Here are some links;
QuietPC website: click here
Overclock: click here