Quote:
Originally posted by Gooserider
the stuff I've seen on PWM supplies suggest that the cure for noisy motors at low RPM is to stick a biggish cap across the fan leads. Apparently this acts to take the edges off the squarewave output of the PWM circuit and gives the fan something more closely approximating lower voltage DC. It might not be able to run as slowly, but it will be less noisy when it does run.
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You can also try to change the PWM frequency: sometimes the frequency will come near the timing of the highest amp draw, and the fan falls in and out of synch because of it.