This is the almost identical unit for Volkswagen Golf MK1 and Scirocco. (check the prints on the case, interior could be almost the same)

Wiring to the electrical headlight servo's. US models are not equipped with the euro style adjustment feature left of the driver seat.

the height adjust motor in the later 928's works like this:

The system works more or less like a normal servo motor:
a voltage is taken from the height adjust switch, basically splitting the voltage in 3 different levels, and feeds that to the height adjust actuator assembly. A motor in the headlight assembly works as the actuator. The motor moves an arm forward or backward, and an integrated potentiometer takes reading of the current position. A comparator in the electronics compares the setting with the actual position and adjusts by running the setting motor in either direction.

Sudden humming of the setting motor might be caused by contact issues on the potentiometer which cause the servo to get wrong feedback and adjusts accordingly. Some contact cleaner might help here although bad wear or oxidation will not be completely solved.