My outside unit hums even when the unit is not running. I'm afraid it might catch on fire so I shut it off. Any ideas why it hums?

I would turn it back on and have someone turn the breaker off to the furnace. See if the stops the humming. That way you know if it's low voltage or high voltage hum.

bitters... WHAT??
The humming will stop whether it's low voltage or high voltage. Your response is inaccurate.

Missouri, you are right about the voltage. Thanks for the correction. The poster said the unit hums even when it is off, which makes me think that the contactor is still energized. The compressor might have went off on overloads because the fan stopped working.