Every so often, I show up in the office to see that my year-old G5 tower has decided to emulate a jet engine. Overnight, the fans rev up to the maximum, and stay that way until I come in and restart.

After the latest software update from Apple, it also began revving up during work, but briefly, as if I had done something that taxed it.

After reading various forums, I found the best solution seems to be to open System Prefernces: Energy Saver, click on the options tab, and look toward the bottom of the panel for "Processor Performance." For me, "Reduced" performance seems to work best, but other people find "Highest" works better for them. Apparently anything but "Automatic" is likely to reduce the occurrence of fan revving.