Switch the rear end lube to synthetic as see if that helps. Also, is it contant, or only during "float" condition -- that is, constant speed with little or no acceleration/deceleration.
Likely it is some wear, but I'd not tear into it unless it is so bad you can't stand it, it's gonna be expensive.
Mine has a whistle too, about the same speed, haven't changed the lube yet, but I've been told it's not worth the money to fix it.
Verify it isn't the tires, too -- I don't remember hearing it so much after I put new tires on!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!