I would add some oil to the constant velocity joints on the rear axel. It sounds like one of them may be warming up and clunking. If this is the case you will seldom if ever get the noise on short runs. Not enough time to warm one with low oil lubrication up.

Check the joint boots for integrity and any evidence of oil leaking externally. It will not hurt to remove a single use clamp. Use a very large syringe to inject some heavy oil and apply a normal clamp to close the boot again...

These intermittent thumps are common on all types of cars with cv joints. Right now it sounds like you are just getting a warning to lube one of them. When the problem becomes constant it is then too late. The joint is shot. Remember they may still be the original axles and have not seen any attention since new as they are usually very long lasting.

I have personally done this on non mercedes cars I owned and got another 50k miles or more. I state more because I either stopped driving them about then or sold the cars and they were still quiet. You have to catch the fairly dry joint early though. The price of procastination is replacement. Unless you locate something else pretty fast thats what I would do.

Now for the interesting senario. If axels are removed and properly drained then relubed with the right quantity of lubricant. Then installed on the opposite sides you then almost have a totally new wear area in those joints to start on.

The same rule applies to getting a used axle. Get one from the opposite side of the scrap car. If you follow this procceedure a used axel is in most cases far superior to a new chinese fabricated one. Remember in most cases an old mercedes diesel is not scrapped because a rear axle cv joint has gone south.

Usually when a fairly dry cv joint starts thumping it does not stop by just changing the speed a little. You usually have to stop and let it cool down to stop the thumping. On that basis it could have just been your exhaust pipe. Yet that pipe thumping should have still happened on shorter trips as well. If it comes back even once again though remember this post. All cv joints do not always follow the most common behaviours in my opinion.