You could try a fan controller to lower the speed of the fan(s) if thats where the main source of noise is coming from. You could also try buying some quiet fans (Panaflo fans are amoung the quietest and can be found here for a pretty reasonable price
http://www.jab-tech.com ).
There's also other retailers that specialize in quiet PC's:
http://endpcnoise.com/
http://quietpc.com/