How do you quieten down a cooling fan in an Alienware Aurora desktop?

I have an Alienware Aurora which is driving me nuts right now. Everytime I turn the machine on, the cooling fan start making this loud humming sound. It sounds like a jumbo jet about to take off. I tried cleaning off all the fans with dust remover...it didn't fix the problem. I also checked to see if there's anything loose...there's nothing I could find. I know there's another option under BIOs. If anyone knows what settings to change, please let me know. I don't want to screw up my system by choosing the wrong settings out of stupidity. Under CPU Fan Control Settings - It says enabled. Under Fan Control Settings - It say enabled as well. Under Lowest Fan Speed - It has it listed as slow. Do I need to change these & if so, which ones? I can't adjust the RPM speed & temperature settings. It won't allow me to do so. Any help would be appreciated because the sound is annoying to listen to. After each use, I have to turn the machine off because of the loud humming sound.