DIY, Changing Rear Struts in a 95 S600 with ADS and Self Level
 

Great post
 

When your rear strut began to make noise, was it consistent, or intermittent?
I've got something in the left rear suspension of my 96 S500 Coupe with self leveling struts that makes a strong groaning/rubbing noise with rear lower control arm motion. Doesn't do it when first starting off, but occurs pretty consistently when the weather is hot and the car has been driven a fair distance (10 miles or so).
If I reach under the car while someone bounces on the rear bumper, I can feel the strong vibration in the lower control arm when it makes noise.
As luck would have it, I spent some time this weekend trying to find it, but it was cooler than normal with rain... drove it several miles, but couldn't make it groan/rub. You can bet it will be back tomorrow. :)
Checked the rear hub for play but there is none.
I'm betting its the joint where the hub meets the lower control arm, but after reading your post, I'm wondering about the strut...
Thoughts?

Originally, I did not notice the sound. My son heard it when he was riding in the back seat. He heard a "rattling" up near the headrest area. It took a few more weeks before I could hear the noise from the driver seat. I also tend to drive with music on all the time, so that helped mask the noise. My noise did not sound like groaning. The noise sounded more like something was loose and bouncing around. I took the trunk liners out, and found a couple things loose that could easily have explained the noise. I secured those items, but I still heard the noise. My noise was very consistant. I heard it immediately when pulling out of the garage and driving over dips and bumps. It seemed to occur whenever the suspension was compressed, and occured cold or hot. I then thought it might be the bump stop on top of the strut. When I removed the strut, I found that the bump stop threaded shaft was slightly loose within the body. I thought for sure that was causing the rattling. I replaced the bump stop, and as soon as I pulled out of the garage, I heard the old noise again. I did what you did, and grabbed various things under the car while my son pushed up and down on the car. I definitely felt something going on inside the strut. It felt like the internal seals were sticking, and as they broke free of their "sticktion", created a creak / subtle snapping noise. If you can, get your hands on one of those powered earphone sets that have a microphone pickup in a probe. The probe can be placed on each component. Listen for the noise while someone moves the car. If you can't find a set, use a long piece of metal or tool, and place the tip onto each part, and your ear on the other end. This can also help pinpoint the source of the noise. You could also remove the strut from the car, and try moving the car up and down to see if you still have the noise. That's a good deal of work, but the strut is expensive. It would be a shame to spend the money on a strut when it might be something else. Hope this helps. Let me know what you find.