When sitting in the drivers seat of my 94 Explorer, I hear a humming noise coming from the dashbord (LEFTSIDE). I shut of all accessories (radio, heat, wipers etc..) I don't hear the hum when I open and check under the hood. I only hear it when sitting in the vehicle. It is a steady humming. When I Rev the engine, it remains the same tone.When I put it in drive or reverse, same steady tone. When I turn the ignition on without starting the engine, there is no hum. I checked the washer fluid pump, but it was working correctly as was the wipers. Any suggestions on what is causing this sound would be appriciated.
Thank you