So my heater stopped working a while ago, and about the same time, my engine compartment started humming when in vaccume (namely at idle and right when i shut off the engine as the manifold pressure returns to zero). This leads me to believe that the valve that opens to either let coolant flow through the heat exchanger or the valve that switches to send air through the heat exchanger is either broken, or the line going to it is leaking air.

Ive never really dealt with heating or cooling systems in cars, so does anyone have an opinion?