Fan Runs Indefinitely

I have a Quicksilver G4. When I go into the Energy Saver and configure my computer to automatically go to sleep and put the display to sleep after 15 minutes of inactive, only the display goes to sleep. The fan in the computer continues to run indefinitely. Alternately, If I put the computer to sleep manually, only than does the fan stop running and the display still goes to sleep as manually instructed. Will someone tell me what is going on? I don't want to work the fan to death If I don't have too. Plus, The Quicksilver G4 is in my bedroom. The sound isn't to intrusive when I am about to go to sleep, but it is noticeable.