It could be anything within the tower. There are fans, drives, etc. The easiest way to determine where the noise is coming from would be to power the system down, open it up and then detach all of the drives. If the noise goes away, then the chances are it is a drive. If the noise goes away, you could then reattach the drives one at a time to see if the noise comes back. Once you determine where the noise is coming from you would probably want to contact Dell for additional assistance.

This is most often from a fan going bad or, very, very dirty.

Dan