W124 '87 300D Shift "Clunk" Near Rear

Does anyone know why when I accelerate from 0 - 15 quickly or normally, but then left off the accelerator completely I get a loud "clunk" sound/sensation coming from the rear of the car, like near the
differential, as the car shifts itself to the next higher gear? If I accelerate normally from 0 - 60 mph it will not do the clunking sound (and shifts fine through the gears).
1987 300D Turbo W124 Automatic, 250,000 miles
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Probably because your flex discs that link the tranny to the driveshaft is shot. Diesel engines are particulary hard on these discs.
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If that is correct that it is the flex discs that are causing the clunk sound, what would happen if the problem is ignored in the long run? Will it eventually break and leave me stranded?

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The clunk is because the play in the flex disc... when they take it out. Have them show it to you... new is solid piece together with that sleeve for bolts to go through... when they break, the sleave is separated from the rubber disc.
The part is not expensive... labor is not expensive either. Long run damage would be transmission seals at least the rear seal... I don't know how much damage it will do to your tranny internals... with vibrations... can ruin the clutch packs and seals.
Then there is your center bearings that ties the two driveshafts together...
Lastly, the vibration will damage the rear end seal also.
Lastly, yes, it will leave you stranded if it destroyed the flex disc bad enough With hard clunks that you can't ignore... but I doubt it will get to this stage.
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