1) The sound needs to move: Attach the ambient_generic to a func_tracktrain on a path_track across the top of the map. 2) The sound needs to have a Doppler shift. This is the change in sound pitch as it goes by. The neeeee-awwww part. Hopefully this is in the sound file you have.
These must be in the main map, not in the 3D skybox. However, the plane itself could be in the 3D skybox, and therefore you'll need a func_tracktrain for the plane as well, and synchronise the two. Remember the 3D skybox world is 1/16 (0.0625) of the scale of the main world. So the speed of the Train needs to be 1/16th of that of the ambient_generic's sound. Trigger them with the same logic_timer, and should be no problem.