I like silent computers. (apart from the speakers that is)
After my generic 80mm case fan nearly drove me crazy I cut out the "fan grill" that is punched out of the back of the case with a pair of metal snips. This grill must have been blocking half the airflow from the fan because now the fan is almost silent and the m/b and cpu temps are down by a degree or two. I also mounted the fan with rubber plumbing washers ($0.20 for 4 at the local hardware store) between the fan and the case
I'd recommend removing your m/b if you do this so you don't drop any little pieces of metal down onto it. Also you should file the places where you cut smooth so you don't cut yourself.
You can use the same technique to make power supply fans quieter too if your power supply just has holes punched in the sheet metal and no real grill.
Just don't stick your fingers in the fans when reaching around the back of your case.