Mac Pro fan makes constant noise. Why?

Solution 1:

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.

Solution 2:

I don't know if this was really the solution but it seemed to work.

I opened the computer, took out the CPU board and blew the dust off it and the fans and the entire computer. Apparently that did it. Maybe I should do that once a year. The last time I cleaned the computer was last year when I put more memory in.