Sounds like the motor, not worth changing only the switch as the windings may be damaged by the motor not running. That's my guess from your description. Especially since it will hum when the switch is out of the circuit, the puzzling thing is you say it hums when the timer is in the off position? There wouldn't be any power to the motor when off, making it the timer and motor. The motor switch would have to be in the run position to make it heat, and the timer would have to be making contact to make it buzz/run when off. So looks like both timer and motor.