nomad100,
Below is a link to the ford parts catalog.
Just enter your VIN and it will bring back all the ford parts for your chassis.
Hope this helps.
http://www.fordparts.com/Default.aspx?gnav=nav:home
The motor you hear, might be the pump for your air horns, not sure. I have one (not working) on the drivers side just above the normal horns.
If yours is working, you might want to check the switch on the dash for your air horns and see if it is stuck open, also, you might have an air leak.
The pump should run long enough to fill the air reservoir (black pancake looking item) about center of the engine bay. Once the reservoir is full, the pump should cut off. BTW, you should see an air fitting (QD) on the line coming out of the reservoir .
Just a thought