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?