85 300d Mysterious rear knock after 1 hr

After driving at highway speed for about an hour and 15 mins, the rearend starts knocking. I can stop the car for at least 5 mins, the knocking goes away until i drive another hour. Then the process starts all over again. Anyone have any ideas?

Thanks

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Diesel Engine Specific Aid

Chassis Model Number

W123

rear knock

I have similar problems in my '85 300TD wagon. It got really bad for a while so I had the u-joint replaced. STILL had noise for a while...then it mysteriously disappeared for months. Now it's back. Was about to go back to the shop - figured the driveshaft was improperly balanced - but now, after scouring lots of forums, I suspect the rear axle half-shafts.
Here's what I'm going to do:

1. Put it in reverse. Apparently, when those half-shafts wear down, it's from going forward. Backing up a bit sorta "resets" the things and you can get a few more miles out of 'em. Not a fix. Just a stopgap.

2. Check the rear CV boots. On cars as old as ours, I can almost guarantee they're ripped. Pack 'em with grease. Again, a stopgap.

3. Check the rear swaybar stabilizers. These wear out, and are cheap to fix.

4. Cough up the HUNDREDS of dollars for the half-shafts.

84 300CD rear knock after one hour

I had the same problem with my 300CD. The rear half shafts had cracked boots and were original equipment even after 325k miles. I coughed up the cash to replace both half shafts and the problem has not returned. Unfortunately, it was a costly fix.

84 300 TD wagon rear knock after one hour

I also had the same problem and found the right rear half drive shaft bottom boot was torn. Cut the rest of the boot off and went to the o'reily's to buy a sliced boot that comes with screws to hold the boot together. that way one doesn't have to remove the whole drive shaft. Unfortunetly it didn't stay on there very long. I think the boot spins and the screws caught on the a-frame and ripped to shreds. But also where the shaft meets the back of the wheel, bearings are visible and I just packed a lot of thick grease into it. I carry the grease with me and repack it when it happens, which is about maybe 3 thousand miles.(approx)