Ok, I am going to start a new thread about my earlier "questionable noises from the rear" thread.
My 81 240 driveline starts vibrating noisily that is both heard and felt through the seats and floor in a percieved rotary fashion {as compared to a bad wheel, tire or balance issue} that begins to be felt at about 25 mph and peaks around 55.
I have replaced wheels and tires, center carrier and bearing and diff lube
all to no avail.
I notice a clunk in the diff area when I shift and release the clutch.
I was told by my mech who both road tested and ran it on his lift, that there is a worn rear wheel bearing but that the vibration is not being caused by that.
The axles are original and the boots are intact though weather checked.
I am leaning towards a worn differential or related such as the pinion bearing but have no hands on experience with pnion issues.
Could such a vibe be made by the pinion, bearing or both and how can I check this.
When I had my DS out I pulled and turned on the diff flange but felt little or no rotary or axial movement.
I am driving every day and wonder if I am looking at imminent parts failure or will this linger and get noisier and vibier long before catastrophe
strikes.
I hope the latter is the case because I park the car all winter and can tear it down at that time.
I really have been trying to approach the problem logically but could use more input
Thanks all, I know in time the answer will be found

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily

There was a proceedure years ago. Still valid. Take a couple of screw clamps and install them on the driveshaft. Rotate them to different positions and test drive. See if you can modify or eliminate the vibration.

Since you are in no particular rush this could lead either to establishing and correcting an out of balance driveshaft. Or eliminating it as the cause. Also really inspect your rubber disks carefully.

I do not know if your year has the factory alignment marks to pair up the two piece driveshaft properly. The earlier versions did not have them.

Perhaps someone unaware that the shafts are balanced together removed them at some time and did not observe the requirement to mark them for proper re assembly. That error can certainly cause this kind of problem as well. You would not be the first.

Yours truly marked both DS halves at the splines and the end flanges before removing it for the center bearinng R&R. At that time I got a look at the flex discs and they looked good

__________________
1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily