Does the pitch of the noise follow the speed of the rear wheels? Meaning, if you increase the indicated speed does the pitch get higher? Does the volume get louder with increased speed?
If the noise is a high pitch, it is not likely something heavy and large in diameter, especially if the pitch is high when the noise starts. Is the noise loud and obnoxious outside the car, where the pitch was noted to be a groan/humming sound instead of the jet noise? Have you disconnected the speedometer gear? This could be something that is capable of generating a high pitch. About the only other thing I can think of that might make a high pitch even at low rpm would be something externally rubbing on the drive train that is attached to the body, and you are exciting a resonance. Maybe if you go to a place with a lift and you get all 4 wheels off the ground, you can get under there when the noise is present and try to pinpoint it.
Good luck, hope this helps, Jim
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
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)