Answer
The buzzing you hear is the solenoid pack that is
part of the 41te automatic transmission. rather than using hydraulic pressure and mechanical valves to engage the transmission and control shift points, the 41te uses a computer and electric solenoids. this solenoid pack is located on the outside of the transmission, near the radiator, and is covered with a black insulative cover. an 8 prong electrical connector comes out of the top of it, and sends the electric signals to open or close these solenoids. the buzz is most noticable when you put the vehicle into reverse or drive from park.
hope this helps. enjoy your minivan.