Could just be dust as was previously stated. It happened to one of
the DECstations I was keeping an eye on that had been running 24/7 for 7
years and was never cleaned. I had to take it outside to get the dirt off
with a can of compressed air and some kind of anti-static brush.

	I believe that the psu fan has some kind of temperature or
rotation sensor system. If the fan is not spinning fast enough it will
kick off. I think there's also a thermal sensor in the CPU itself, but
NetBSD doesn't monitor it I don't believe. I'm probably spouting
conjecture more than actual fact, but hopefully that gets you on your way.

-- 
The nice thing about Windows is - It does not just crash, it displays a
dialog box and lets you press 'OK' first.
						-- Arno Schaefer