What does hum mean? It can mean two things. The first, on older designs, the motor may be running free and the belt is broken; no belt, no turning. This can be confirmed by turning the drum by hand and checking for drag. One with a good belt is hard to turn. You are rotating the motor with your hand. If the belt is broken, the drum rotates easily. Get busy with a belt and idler job. This is not the case on dryers with a belt check switch. They not hum or make any noise. See "Dead Dryer."
Broken Whirlpool Belt
Second, and much more likely, a hum can also mean the the motor is bound or too weak and unable to turn. This hum doesn't last long before the motor overheats and quits trying. (If the hum sounds bad, don't leave it on for more than a few seconds. The motor will overheat, quit and will prevent futher diagnosis.)
Bad Motor Noise
With a loud unpleasant hum, you must detemine if the motor is locked up (the more likely case), too weak or whether something has jammed the blower.
If the motor is locked up it will be very difficult, just barely possible to rotate the drum. The belt must slip past the motor pulley drag in order to move; this requires a lot of effort.
If you determine that the motor is in fact turning, hums, but will not start up on its own perform the following:
Trick of the Trade:
Using three hands, push the start switch, depress the door switch and rotate the drum; all at the same time. A weak motor will start right up and run fine, if you can close the door really fast. This is a positive test for a weak motor; replace it.
Locked Up Motor
It is hard to quickly tell whether a jammed object or locked motor is the root cause; they both have the same exact symtoms. To be certain requires disassambly. Unthreading the belt and attempting to turn the motor shaft may provide a clue. If you can freely wiggle the shaft through part of a rotation, the chances are something is caught. Objects like socks, combs, brushes can all get past the lint trap and get into the blower. If the shaft is locked up solid, chances are the bearings are locked and full disassembly and a new motor are required.
Caution: The Whirlpool blower is famous for getting jammed by a felt seal (a piece of the motor shaft seal) that gets stuck between the blower and the housing. This felt can make very unusual and misleading noises. Many a tech has put in a new motor to find this mysterious piece of lint all balled up in the blower.