moaning noise in 190 E

On my 1988 190 E 2.6L 5-Speed I get a moan at around 35 mph and it gets loader until, at 80 mph, it is a roar. The odometer reads 249,000 miles. The car had two new flex discs and a drive shaft center support installed the other month when a new/used differential was installed. Red Line fluid was used in the differential. The rear CV joints were inspected and found to be in good shape. Tire pressure is 36 psi all around and the tires are in good shape. I cannot locate the exact source of the noise, its from the back end somewhere. It is not a chirp nor a high pitched sound, it starts as a rumble-moan kind of noise and just gets louder as speed increases. Last week- end I took the snow tire/wheels off and put the street/summer wheel/tires on. I was hoping that the noise was just the sound of the snow tires (thats what the noise is like - very noisy snow tires). It was not. The noise is still there. Could it be the rear wheel bearings or would they make more of a chirping sound if they are bad (like front bearings can chirp when they need grease)? Any ideas? Thanks for your time.