The speed control is thru the BIOS, and Dell doesn't let users adjust these settings. That's why you will hear the fan speed cycling from low to high, depending on the CPU temp. Same with clock speeds; not adjustable.
In case you are experiencing some rumbling noises, place your palm against the case. If the noise goes away, then the noise is caused as a result of resonance. Place a couple of refrigerator magnets on the side of the case, and that will solve the issue. Do not use heavy powerful magnets; will create problems with magnetic media such as hard drives.