When decelerating there is a knocking sound from the center bearing area. The center bearing and rubber were replaced three months ago so I rule that out. Under load the shaft is quite. When the foot comes up it's knoka, knoka, knoka. At drive shaft speed and more noticable at lower speeds like comming to a stop. Not the sound a bad pinion would make either. Could this be the universal joint? How much slack would there have to be to make this noise. Would the slack be visable where the shaft enters the bearing cup? How do you test a universal joint?
Edit: the flex disks were replaced along with the center bearing. There is a vibration damper in the front. All the bolts are tight.

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1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

I still recommend taking another look at it and be sure....I've had the rubber part fail in less than 2,000 miles and less than a year. Apparently it was a really crappy quality....the next one has been on it for nearly 9 years.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

X2; the troubleshooting Gods say that whatever you think is OK and do not check turns out to be the cause of the problem.

On my Volvo I jacked the rear Wheels off of the ground (safely) and put it in Gear.
What I saw was my very similar support Bearing jumping up and down.

In one of the Threads they said the car has to be setting on the ground and rocked back in forth before the Collar (Coupling Nut) on the Drive Shaft is tightened (on a 123 you need two 46mm Wrenches).

I could not see any thing wrong with my U-joint until I removed it and found it was very stiff.

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84 300D, 82 Volvo 244Gl Diesel

Ya, check that bearing. And the bolts that hold that thing to the car. It takes a lot for that U-joint to fail. My custom driveshaft guy said the driveline is pretty straight in a MB and not to worry about the U-joint. If it is indeed "bad" then you need to find a used rear shaft. I think only one place in the country will replace the joint, and it's NOT cheap. But nothing really is these days.

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08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D

I had a weird growl from my driveline that I couldn't find. Put the cat on jackstands and let it idle in gear... thump thump thump was coming from the rear diff. If you do this be careful... I was under there listening to stuff with a screw driver .

But I have done maintenance on a helicopter with the rotor blades turning.

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08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D