dwcasey,

The humming can be the differential as you have noted, or it can be the center support bearing on the drive shaft. The center support bearing is not something I would do as I have limited tools and no lift. I have had it done on cars in the past for around $100 at an independent mechanic's place. That was a long time ago though, so it might be twice that today.

I would change the differential fluid and the vent on the top. I would put in some Redline (www.redlineoil.com) "Shockproof" gear lubricant in and see if the problem persists with the humming. My experience has been that this stuff (the heavy formula, anyway) changes the acoustic response of the gear meshing and unmeshing, and really tones down a differential. Yours, if there is leakage around the unit, may be leaking and have little or no fluid in there.

If the car is in decent shape you will get at least another 160,000 miles of good service from it for $1500 or $1700. I agree with another comment that there are relatively few potential customers for this car, so if the guy really wants to sell, he will likely be willing to deal. Good Luck, and enjoy the car if your negotiations are successful. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)