Furnace blower making consistent loud humming noise. My furnace is a single speed unit, I manually changed pin configuration to adjust blower motor speed. At medium speed is where the humming noise is. At low or med/high or high speed setting no humming and smooth, its only at medium speed this humming occurs. Reason I want medium speed is that at low speed furnace reaches temperature limit switch and cuts out (not enough air flow through burners and units overheats). At med/high or high setting heat balance in the house is affected, so want something between, hence medium! Having said that, if it was a rotational imbalance of the blower drum, my guess is it would have happened at one of the other speeds, no? Unless I really was lucky (or unlucky) to hit that frequency ONLY on medium setting! One other thing to note, that when the blower motor starts up, in all settings, there is that initial hum (probably startup capacitor) then fades away. However in the medium setting, it doesn't appear to fade away. Not sure if that is any clue. Any suggestions/thoughts?

I would start by checking/changing the run capacitor. If that fails to do the trick, it sounds like the medium speed winding in the motor is on its last leg. Only option at that point is to get a new motor and matching run capacitor.