Originally Posted by
arus2001
It's sounding electrical to me, too. When I had my older car, I noticed that when I had the key in position for anything to draw power, I'd hear a faint buzz, myself. My guess is the particular sound is tied to the triggering of lights relative to the gear position, since going in park and reversing with the key on should be triggering the back lights. Unfortunately, light wiring has multiple connections where anything in between could be corroded/frayed due to age and possibly exposed to some bare metal where various wires may also have been bundled against with plastic ties along the frame. Mentioning the dash would then point me toward the steering column portion, where if you have an exposure issue, it could also mean the insulation at the back of the truck's hood near the windshield could need some work to prevent water from drizzling down due to rain/snow.
Given we're talking a 30+ year old truck, that'd be my starting point, anyway.