djwhite,

Well, if it is not a wind noise, a few mechanical things to look at are the center shaft support bearing, or the differential. The center shaft support bearing on a 240D is a ball bearing that is sealed "for life" with a coating of a rubber like material, and bonded to a bracket that bolts to the car frame. Once the rubber stuff gets old and cracked the grease leaves and water gets in, and the bearing will start to get noisy. Bearing noise can be very similar to the wine glass tone you hear, as you can excite the rings into a resonant condition. The noise will seem to get louder or fade if you turn your head. You can check this by visually examining the center shaft support bearing, which is on the driveshaft at about the center.

The differential can whine for some of the same reasons. There is rubbing contact where the teeth mesh, and there are ball bearings (usually) in there. IF the oil in the differential is very old, or the level is low, this can lead to wear that can cause noise for similar reasons. If you have no idea when the differential fluid was last changed, it might be a good thing to prepare to do or have done. Sometimes a fill of good quality, clean lubricant can reduce the effects of the rubbing contact (better oil film thickness) and change the resonant characteristics of the parts enough (like changing the level in your wine glass) so the effect does not happen or is less audible when it does.

There are other sources, such as wheel bearings and stuff in the transmission, but from your description, I would start looking at these candidates.

Good luck, and I hope this helps. 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)