I wasn't unduly worried since everything appeared to be running at a reasonable heat and performance seemed unaffected. However, I thought that I would try to hunt down whether or not this was a known problem—with the recent incremental upgrade I had performed, for instance—and if there happened to be a fix.
As it happens, there is: you need to reset the System Management Controller.
The System Management Controller (SMC) is a chip on the logic board that controls all power functions for your computer. If your computer is experiencing any power issue, resetting the SMC may resolve it. The SMC controls several functions, including:
- Telling the computer when to turn on, turn off, sleep, wake, idle, and so forth.
- Handling system resets from various commands.
- Controlling the fans.
There are a couple of ways to do this, but the simplest—in other words, the one that doesn't involve you stabbing at the motherboard with a pencil—is the following method.
To reset the SMC on a Mac Pro:
- From the Apple menu, choose Shut Down (or if the computer is not responding, hold the power button until it turns off).
- Unplug all cables from the computer, including the power cord and any display cables.
- Wait at least fifteen seconds.
- Plug the power cord back in, making sure the power button is not being pressed at the time. Then reconnect your keyboard and mouse to the computer.
- Press the power button to start up your computer.
My Mac Pro is now back to its usual near-silent operation. The method for resetting the SMC or PMUs for other models of Mac can be found under point 9 in this article.
UPDATE: I spoke too soon. Whilst the brief period of shutdown cooled the machine sufficiently to gain a reduction in noise, it soon started up again. I tracked it down to the video card fan, which was going like the clappers.
After performing a hardware test (no problems) and then running the machine with the side open to ascertain that the main fans were all running correctly, I took a look at the Radeon graphics card. Why didn't I think of doing so before?
The air input for the fan was solidly clogged with dust and other crap (not least, the ash and smoke from about 5000 cigarettes, I'd imagine). As such, the fan was having to spin ten to the dozen to pull any air over the components at all. So, after removing the graphics card and cleaning the entry to the fan airway (I'l need to go and buy a new toothbrush!), I replaced the card and the machine is now absolutely silent.
Now I'll know what to do next time that happens...
UPDATE 2: for the usual commenters, here is the "official" description of our old friend, Artie MacStrawman.
Mac users will remember MacStrawman as the Mac user who:
- Says the Mac is utterly invulnerable to any and all malicious attack.
- Mindlessly worships Steve Jobs.
- Blindly buys anything Apple releases no matter how dumb and stupid and dumb it is.
- Refuses to accept that Windows might be better at anything. Even being Windows.
- Emails death threats to anyone who disagrees with him.
Daring Fireball's John Gruber said "I just wish that guy’d switch to Windows or Ubuntu or something.
"But... he's Artie MacStrawman. So I guess that’s not going to happen."
I would also like to add that Artie MacStrawman also maintains that Macs never, ever go wrong (in defiance of all physical laws) and that Macs never, ever need a tune up or a bit of desultory maintainence from time to time.