I have a 1997 Ford F-150 S/C 4x4. For a year now (as long as I have owned it) I have always heard an electronic "humm" noise from the underside of my truck when the engine is OFF but the key is in the on position. I can only hear it from the outside of the vehicle. I have always thought it was the electric fuel pump in the gas tank. However today I went under the truck to see once and for all to come and find that the electronic "humm" noise was coming from inside the transmission. Is this normal a noise for the eletroniclly controlled 4 speed auto tranny with O.D? It does shift fine and allways had. It is the 4r70w series tranny if I typed in the correct letter/number sequence for the designation. Please give me any insight reguarding this matter.