When my 83 300 sd is driven rather hard sometimes it will start thumping right before the silly thing shifts. It is coming from the back of the car. Drive line??? Rearend??? Any Idea? And now it also shakes at certain speeds on the freeway, when you let off or take it out of gear it stops very quickly. It has a bolt missing on the left engine mount and I know that it is a problem but I am going to fix that and it came up missing after the first problem started. Thanks.
Hudson

That missing bolt is your best bet.....

especially if its the alen bolt, better replace ASAP!

Look at you Flex joints for damage, front AND rear, look at the driveshaft center support, and look at the rear differential mount too. its one of those most likely.

__________________
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

missed that.................yes get a bolt in there.

__________________
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)
---------------------
Section 609 MVAC Certified
---------------------
"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

As I am sure several others will tell you...
I'd recommend getting under there and looking before you drive it anywhere else.

Take a good look at the rear flex disk. As well both axles.
Your flex disk may be failing. If it breaks at driven hard speeds your driveshaft could really tear your car up.
You might also have a torn CV boot that is flopping around.

Regardless, a thumping noise like that is not normal and ignoring it could wind up costing you alot more than a repair bill.

__________________
'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.