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