Steve,

If the noise is consistent, happens every time you load the wheel up in a particular way (speed, turning radius) the problem is likely wheel bearings. These are a replaceable part and are not a high dollar item. For the most part if you are hearing noise only in certain circumstances, the wear is just getting to be a problem. It will not likely leave you stranded in the near future, but it is something you need to fix as it can lead to control and safety problems. I have seen a rear wheel bearing sieze (the fellow left his e-brake on since the dash light did not work, and drove about 120 miles on the highway), and cause a lot of collateral damage. So, I would get it fixed, or fix it myself pretty soon. Good Luck, 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)