This tool allows you to manually control fan speed: http://www.almico.com/sfdownload.php
If your fan is flying due to the CPU load though, might not be the best idea to throttle it for noise. You can play around with it though. Your CPU should kill your computer if it gets too hot (it'll blue screen with a CPU watchdog error or something) to prevent damage to itself. You can monitor temps with that program as well.