MerzBenz 300D Occasional speed related clunk

Occasionally when driving hard and fast, or loaded and fast up a steep hill, the car will develop a speed-timed thump that will develop into a definite clunk after a few minutes of driving. If I pull over and back up the car a few feet and stop, it all goes away till the next episode, which could be in 10 minutes or 10 days. What is this!

How many miles do you have on this fine machine, what year is it?

I had a similar intermittent clunk on my '83 300D, and it turned out to be a worn out engine mount on the passenger side that was allowing the engine to wobble about 1/4" resulting in the engine constantly lifting up and thumping back down on the rubber. Another common problem can be broken air filter housing mounts that allow the air filter box to wobble up and down. You can see this easily at idle with the hood open, apply some throttle and see if it vibrates around or stays still.

Do you have access to a vehicle lift? I would lift the vehicle and check all the rubber driveline components. Check the front steering links and their rubber boots just for good measure. Moving back, look at the engine mounts and check for clearance with a pry bar, look for dry rot cracks at the propeller shaft rubber flex disks (if a flex disk fails at speed, you are absolutely phucked… it will violently tear up the floor pan and cause all sorts of mayhem), and lastly check the the boots on the rear CV shafts.

Thanks W123. Have looked at all the things mentioned. This develops into a loud metallic clunk. The frequency slow and increases with the speed of the car, so not an engine mount. I’m beginning to wonder if it’s the tranny?

Well, I’m hesitant to really blame the transmission yet because these things are pretty simple planetary setups that are pretty bulletproof so long as you’ve got fluid in them… getting proper shift behavior out of the thing is a whole other ball of wax.

Year and miles would help.

And you say this only occurs when you drive up hill and while driving aggressive? There are no other conditions with this happens? Not when downshifting or making turns. Then you reverse the car and it goes away until drive aggressively again? Can you pin point where the noise is coming from? perhaps a passenger could help…

Hi binkman, Yes you have the background and scenario details correct. In addition, the car shifts well, and no other tranny problems, but still suspicious. The car is a 300 D 1985 and in mint condition otherwise.