If the whine occurs at a certain speed (my old 300SD's is 40 mph) AND it
occurs either on application of the throttle OR no throttle - coasting -
is a ring and pinion gear mesh issue that can be corrected.
Any good automotive fix it book addresses this adjustment - it usually
involves adding or removing a very thin shim or two so the pinion gear
is deeper or less vs. the large ring gear. The fix it books will tell
you the direction that the pinion needs to be moved according to when
the whine occurs.