"There are two reasons to replace a PSU fan. The most obvious reason is
failure. Perhaps it's making a sound like a sick buzzer, failing to start
all the time, or just not working at all. If you don't fix a broken fan
quickly, you'll have more than the fan failing. The other reason to change
the fan is often overlooked. PSU fans are usually pretty bad performers.
Replacing the old, slow fan with a high performance model can double the
cooling in your case while keeping the component count down."
FULL STORY @ POIT (http://home.carolina.rr.com/poit/psufan1.html)