Mac Pro fan makes constant noise. Why?
In this Apple support article, Apple says that resetting the SMC can fix fans that are constantly spinning for no reason.


  After performing normal
  troubleshooting, these symptoms may
  indicate that an SMC reset may be
  necessary:
  
  Fans The computer's fans run at high speed although the computer is
  not experiencing heavy usage and is
  properly ventilated....


Here's what it says about how to reset the SMC on a Mac Pro:


  Shut down the computer.Unplug the
  computer's power cord. Wait fifteen
  seconds. Attach the computer's power
  cord. Press the power button to turn
  on the computer.