need copy of schematic or help with question. Air works fine. Turn heat on and after about one minute a loud buzzing noise starts. Heat still works while buzzing is going on. This is a down flow unit located in a low, tight space. Hard to see inside of unit. Buzzing is coming from back of inside where the control wiring is. It is either the transformer, contactor, or limit switch on the back inside panel. Hard to tell exactly which part it is coming from