[B and C]

Any ideas on what could cause a humming noise in the upper console cabinet on the passenger side of the cockpit.  There are two circular 2" vent openings in the upper far corners on both sides of the upper front cabinets and there is a hum or fan type noise coming from the right one.  Below is a picture of the back and you can see the vent at the top and a junction box mounted on the outer wall.  I can't get into the area without dismantling the cabinet.  I hear a slight sound change when I turn on the small overhead light in the right side audio cabinet.  Very odd.
Thanks.