wolfv,
I am running my fan control at 25% and the fan runs at about 2250 RPM. At this speed with no activity the D54250WYKH runs at 38deg C. Just ran Intel's Processor Diagnostic Tool and the machine never went above 66deg C though the clock speed only temporarily went to the maximum of 2.6GHz. Most of the stress test the CPU clocked at around 2.25GHz. These temps are "cool as a cucumber." I have a Mac Mini from 2011 and the CPU runs 95-97deg C when processing video with the Handbrake video converter application. I would only worry that the fan may require a minimum pulse width drive to keep from stalling. The hum may be the PWM frequency on the fan drive.
Rick
