Agreed...many bios' come with the fans set to go rather slow. That or sometimes fan just go slower than motherboards expect. My previous CPU fan would set the alarm off now and then because it's max speed was only 1600RPMs.
Or, of the fan in the system's old, it might just be getting worn out. They are very easy to remove and replace. It might be worth just popping the case open and see if the fan feels stiff. If so, take it out and see if your local computer/electronic shops have any replacements.