1996 Subaru Impreza

Hi
i have a 96 subaru impreza outback and my problem is that the car headlights and the inside lights and the dash lights go weak and strong or up and down when the car idling or when in the freeway and it makes a humming noise but it does not affect the driving conditions, and I want to know what the problem is before I take it to a mechanic, to see if is worth pay the repair.
thanks for you time