dxw00d:
PC fans are designed to be driven by PWM at a much higher frequency than the Arduino's default hardware PWM. That is probably the cause of the noise.You can change the PWM frequency - Arduino Playground - HomePage Arduino Playground - HomePage
Yeah, the default's right smack dab in the voice/ear sweet spot of the audible range at 976 Hz. Probably for newb's like me to hear it when we hook up a speaker to it in our "PWM, hello world sketch"! XD
Thanks for the references.