AC is off and a hum can be heard coming from the unit outside. Should I be concerned?

The Contactor may hum when unit is operating, but you wouldn't hear it due to the Fan and Compressor running, I have seen units that break 1 leg of the 240 volts and the compressor has a constant hotleg.
If you can do safely, see if you have a 1 or 2 pole contactor outside. If 2 pole, all power should be removed from Compressor and Fan.
Do Fan and Compressor appear to work correctly, Spinning and cooling?

I have heard transformers (http://www.thermal-medics.com/a-c-image-glossary/) hum from the street below the rooftop unit. Almost like walking under a high voltage power line.