w123 highway clunking driving me nuts
 

After about an hour of highway speed driving there is an erratic clunking underneath the car towards the rear. Initially I thought it was the exhaust clanking around, but I removed the exhaust and it's still happening.

I've read that this is often the rear CV axles. Mine look good, boots are solid. Could still be the issue but wanted some feedback here. It sounds like someone whacking the underside of the car with a mallet. Only when fuel is applied, not when coasting. It's also not exactly rhythmic, random in intensity.

Check your rear flex disc and the mounting bolts.

Also check your exhaust heat shield mounting bolt(s).

While looking at the flex discs look at the drive shaft support.

Is the clunking when you are driving straight or does it only happen when you go over bumps?

Does it clunk when you accelerate from a stop?

There is a large drive shaft coupling nut. See if that is loose.

Does it stop even for a little while if you stop and backup for a few yards ?

Happens going straight, nothing to do with bumps. Wouldn't know about backing up because it happens at highway speed. Does not happen from a stop, only after about an hour of 60+ mph speeds. Sometimes continues for a few minutes after exiting the off ramp, but subsides. I've checked the flex disc, it looks good.

Low lubricant level in the cv joint is a common cause. The cv joint has to heat up before it is obvious. Add some very heavy oil to each side to see if that is enough before too much damage is done.

Just loosen the inner clamps and feed a tube in through the boot to axle space feeding it with something like a turkey baster. Almost 80 percent certain this is your issue.

The longer you drive it like this the higher the odds become of the cv joint being destroyed. The clue is always almost the same. Car has to be driven until the joint components heat up. From the lack of lubrication.

Let us know if this solves it. You still stand a good chance as they are not clunking all the time yet. Once they are clunking all the time it is usually too late.

The original lubricant was heavy oil. Some use grease but if you have taken apart damaged cv joints you will have noticed the grease has lost the contained lubricant and only the soap filler without the oil is left. Usually well displaced from the area where the lubrication is needed. Straight heavy oil does not do this.

Could be just the exhaust hitting the body, which can make a very noticeable sound. My 1984 does that the few times I had two big guys in the back, compressing the springs. Happens mainly at idle. I tried to reproduce it, having my wife stand on the rear bumper as I looked, but couldn't find anything in the exhaust that could touch, so perhaps a worn rubber bushing that allows metal-metal contact under load. The independent rears in these cars is more complicated than the simple solid axle - leaf springs of most 70's U.S. cars.

As said above, sounds like cv joints, since heat build up is a factor.
I've had good luck using long curved, tapered, all plastic-tipped syringes to apply the gear oil into the boots. I then use hose clamps to seal the small side of the cv boot back up.
It probably throws the balance off a bit but I haven't noticed anything undue.
Good to get them before they are worn out. switching the cv's left to right to begin a whole new wear pattern isn't a terrible idea as well if you've the time and energy.

If yo don't use the special CV Joint Oil from MB use Differential Oil on the CV Axles (I believe some have used that when they re-boot their axles).

Since we're on the subject of injecting oil in the boots...
 

can you recommend a type, brand and weight of that which I should use in my 1985 380 SE differential's axles, please?

Diesel911 just gave you the best answer... notice the use of the word ' oil '...

are these syringes stocked at auto parts stores? how much diff oil should i be putting in?