Ok, I finally heard the noise and am more baffled than ever. She had the car idling in reverse (not rolling at all). I watched her and when ever she would shift into N or P the car would make a horrible continuous groaning sound which would instantly stop if she shifted back into reverse or into drive. I had her shut it off in reverse, then shift to park once the engine was off. When she started it again it was back to normal.
1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver
1982 300SD - 232K miles - Wife's Daily Driver
1986 560SL - Wife's red speed machine