It sounds like the system is telling you that the speed the CPu fan is spinning at is too slow. This could be because the 'too slow' setting it too high, or because the fan is too slow.
For exapmple my CPU fan is currently spinning at 3006 RPM (Revolutions Per Minute). If the speed drops below 2000, my system makes a loud continiuos beeping. Just to let me know there is a problem
But i can set the speed for the alarm to 4000RPM. This way it will always get upset, as the fan won't go that fast.
Do you know what speed your fan is actually at?