its going WHoo0o0oo0o WHeeeeeeeeeeeee Mmmmmmmmmm Hmmmmmmmmmm
a. the belt is about to break. is it one of those pain in the ass serpentine belts? fix it fast or all systems will fail.
b. the bushings are worn. it might be loose and rattling on the flywheel.
c. the blades are bent. rear end anyone lately, or have them back into you hard?
d. low on radiator fluid? the fan works overtime trying to keep the system cool. check for loose/leaky hoses.
e. is the solenoid is stuck in the ON position? replace it or it will drain your battery when the truck is off.
what kind of noise is it making? is it going PING PING PING or is it roaring or squealing?