I recently got a 1994 f-150, 351 SCAB with dual tanks. When I push the dash switch to change from the rear tank to the front, a low constant whine starts, and slight vibration is felt just below the brake pedal. When I switch back to the rear tank, the sound/vibration immediately stops. I am assuming the 351 has a mechanical pump which draws from both tanks, and the switch is simply a diverter valve. Am I wrong, or what might be causing this noise?

 

No both tanks have a fuel pump in them. You need a electric fuel pump for fuel injected engines. A mechanical can not supply enough pressure. What you are hearing is the electric fuel pump. You don't hear the rear because you are isolated from it.