300D rear end "clunks & "walks" sideways,
 

My 85 300D makes a clunking noise on bumps that sounds like loose junk in the trunk or the muffler hitting something, but pretty sure that's not it.

Also the rear of the car slides sideways slightly if I hit bumps on a curve.

Tried searching the forum, but couldn't find anything.

Thanks for any ideas what to check.

The rear suspension on these are pretty simply. There aren't too many options.

The noise could be a few things but these are the two most likely:

Shocks
Sway bar links (pretty likely culprit, common issue)

Here is a link to a guide on replacing the sway bar links that includes a video of what a bad one can sound like - spoiler alert, they are noisy for such a small part!

The "walk" you describe sounds like trailing arm bushings.

My best guess from what you describe suggests you probably have a few issues, not just one. Likely your rear suspension would benefit from a new set of bushings, shocks, spring pads, diff mount, and sway bar links. You can do some searches on here using those terms to get insight on what those jobs entail.

Four bushes on the rear sub frame worn can hive you this problem .

Loose shock can cause it too. If you have a wheel leaning wonky your a frame might be rusted.

I completely agree with "all of the above". If you want to solve the "walking" problem, and probably also solve numerous other handling issues and reduce rear tire wear, it is time to replace all the rubber bushings in the rear suspension, and the rubber caps on the rear suspension springs. Consider replacing those springs as well, they do get weak and break, causing excessive camber which will wear out the insides of the rear tires too quickly.

There are two large bushings for the rear suspension located in front of the rear tires, just about under the rear seat. Big bolt goes through the middle of each, they connect the front "horns" of the rear subframe to the body, and when they wear out, the rear end will steer itself around. Replacing these can be a DIY project, but if your car is rusty, it can be a real challenge. Think torches and/or saw-z-all.

Just realized I mentioned a link to a guide but forgot to include the link.

https://www.ifixit.com/Guide/Mercedes+W123+Sway+Bar+Link+Replacement/22884

Trailing arm bushings and subframe mounts solved "ass walk" in one of my W115s. It would kick one way or the other under acceleration and deceleration, and it clunked.

Thanks, everyone,I think I will start with sway bar links.

A$$ walking. LOL. I just finished a fairly major (for me) overhaul of my rear suspension. The only things I didn't touch were the parking brakes and rear wheel bearings. The rear end was shifting about on the highway like a belly dancer. Not doing it now though :P. My shocks were shot and the subframe bushings were too, but the trailing arm bushings came out easily and looked almost new, so I don't think the shifting about was due to the trailing arm bushings. (I torqued them under load and now my rear wheels are visibly cambered positive :( I should have left them alone)

The only problem with throwing the kitchen sink at it like I did is now I can't tell you what is exactly keeping it stable. But I can't see how those little plastic things could cause the a$$ walk very much (but I have been wrong before).

FYI the subframe bushings can be installed with the subframe on the car if you can get the old ones out.

Replaced Sway bar links finally!
 

Finally, a year later, got around to replacing the sway bar links on my 1985 300D. Both were very loose .

The clunking noise is almost completely gone, and the ass walk problem seems to have gone away as well!

Thanks again for all the help on this forum!