Macmini 2012 with SMC Fan Control and iStat menu monitoring fan speed and temperatures and it locked into 5,500 rpm today and would not quit. I removed the plastic base while stilled plugged in and running and was shocked to find a thick layer of dust similar to the lint you'd find in a clothes dryer covering the perforated metal screen. '''I removed the dust by hand and sprayed the rest with compressed air and the fan speed dropped to normal immediately'''.