Buzzing noise from rear pick up bed on 1991 Ford Ranger

Buzzing noise coming from pick underneath the pick up bed like something electrical is running.. There is no cooling fan on the radiator... Noise would happen intermittenly when I had the heat on... Now the buzzing is constant even when the unit is off... What could it bea? don't see or notice any electrical components near transmission or converter.. help pls

Asked by for the 1991 Ford Ranger
the only thing that i can think of in the rear of the truck is the fuel pump. when the motor goes bad, it makes a ton of noise before it fails. have it checked to confirm.

Thank you.. I will have it checked out