I'm having a problem with my Roper dryer and don't know if it might be the motor or timer.

Sometimes when I turn the timer knob to a setting to begin a load(any setting or temp) I get a loud humming noise from the motor, even before pressing the start button. When I turn the knob back to OFF, the humming stops. I can usually turn the timer back to OFF and back to the setting a few times until it doesn't hum when turned to a setting and then press the start button and the dryer starts and runs fine.

Which is probably the problem? The motor or timer?