Now that we have ACC means accessory out of the way lets anwer that second question "Also I hear some humming noise in the back of the car, I think its the airbags. Does the humming mean that the airbags are on or off?"
Depends what position your key is in. If it happen when you turn it to ON, but not in ACC, then I would say it is your Fuel pump humming. But if it happens in both positions might just be the Electric Antenna motor running.
Lastly, if your talking about your airbag as in the steering wheel and the passenger side... If those are humming I don't think I would be seating behind them. But, If the car was equiped with air bags for cornering, it could be the compressor running.