Best way is to replace the CPU fan with a
liquid-cooled system. These use mineral oils (not water) to cool the CPU, so the need for a fan is all but eliminated.
http://newegg.com has some decent ones, and replacing them is pretty easy. If you're running a stock PC system (like one from Best Buy), your case might not support additional fans, however.
Next-best way is to find a standard CPU fan that is known to produce little sound. They don't get much better than stock fans, however. You would benefit from cleaning out the current fan so that the motor can run smoothly. Buy a can of compressed air from any electronics department (they're like $6-10), unplug the computer and open the side of the case. Blow out all of the hardware components, especially the CPU fan (usually right in the middle of the motherboard) and the PSU fan (where the computer plugs into an electrical cable), as well as any extra fans that the case might have.
Another way is to decrease how much CPU the bots are running. In nexus, find the Global Settings tab and turn them all the way to the right. In RSBuddy, enable
both of the Use Less CPU options. Using less CPU means it produces less heat, and the fans can run more slowly.