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I seem to recall from the time I spent futzing around with a G5 Xserve that the fans in the G5 require a PWM signal for speed control, and that they'll either not run at all or run full-blast (at an ear-splitting volume) if you just apply DC voltage to them.

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Yes, most fans do use speed control these days, and PWM is the obvious choice. That's some upshot of my electronic music experience, setting up a 555 timing chip as an oscillator with pulse-width control is something I have done before. A trimpot for speed control is probably sufficient.