Desktop or laptop?
Have you had any other recent problems?
Has it been moved lately (desktop mainly)?
What makes it sound like it is on if not the fan(s)?
Has the machine been opened (before or after the problem started)?
Was anything disconnected, reconnected, or added new before the problem started?
Was there any recent updates or additions to software or hardware?
Since we are not there if front of your computer (you are), we need you to give us all of the details and be our eyes and ears in order to help you.
You have to be a little bit crazy to keep you from going insane.