I've had this problem for a while, but it's getting louder. Under load, accelerating or decelerating, there's no symptom. If I unload the drivetrain by pushing slightly on the accelerator, for instance when coming down a slight grade, I get a consistent thump. On a long downhill grade like on I-70 coming down into Denver, I can get the backseat passengers to think there's a helicopter hovering over the right rear of the car. The noise is definitely coming from the right side so I'm almost certain it's not the flexdiscs. When I jack the back wheels up, I can get a slight top to bottom wiggle on the right wheel, but not the left. I'm thinking it's the right rear wheel bearing. (Another member had a similar problem on a TD in the past) Got a quote from Startech, the local independent MB shop for $410 parts and labor. This seems reasonable to me. Is my hypothesis correct? Opinions?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Need more advice. Dropped it off at Startech today. They were unable to reproduce the noise. This does not surprise me too much since the conditions have to be precise--downhill grade and just enough throttle to unload the drivetrain. Plus, it's real windy today so it would be very hard with hear with the wind gusts being what they are.
Anyway, on the lift, they can detect no bearing noise but they think it might be the axle joints and are recommending replacing the axles with rebuilts. They say the grease in the axles is dried up. What does everyone think?
They say there is no detectable difference between the two sides of the axles. The noise is definitely on the right side with no noise at all from the left. The noise has been there for at least a year and is slowly getting louder.
Would bad CV joints only show up when unloaded? Why does the noise only occur unloaded?

I'm not inclined to replace the axles without a clear diagnosis. Anyone know how to draw the distinction between a bad bearing and bad cv joint?

Here's a thread where an 85 300td has the same symptoms and it was the bearing:

1985 300TD drivetrain noise

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

1984 300d Turbo
Replace Two Each Axles New Shop Was One Was 725.00 Total, $525 New Axle Labor $200.00
The Other Side New $325.00 Diy 6 Hrs Go To **************.com
For Guide Book $12.00, Very Detail Guide Book