Mine was humming last year. The fan wasn't turning but I could turn it freely with a screwdriver. I called the repairman and it was a bad capacitor. It was bulging and obviously bad. It then ran fine for about a year and started doing it again. I looked at the capacitor and it was fine. I called the repairman and he came out and this time the motor needs to be replaced. A bit more expensive this time around.

First thing to check is to see if you have electric out there. The hum is probably the 24 volt relay that is telling the a/c to come on. The 24 volt relay gets it's power from the indoor unit.