pcmaher is right... these hydro-pneumatic struts do fail internally. Chased a low frequency groan/squeak in the left rear of my W140 for quite a while. Tried isolating all the various joints, but when I put my hand on the strut while someone gently leaned on the back bumper, I could feel the strut vibrate with the noise. Finally concluded that it was the strut... replaced it last week with a new one and noise is gone.
Funny how things work out... the consensus of the threads on this forum led me to understand that many things on a W140 fail due to the complexity of the car... but the struts almost never fail. My personal experience, with a data set of one, is that the car is extremely reliable, but the hydro stuts are not.
Now the bad news. If it is your strut, they're not cheap.
pcmahar: I read your webpage and followed your process. Not a bad job overall. Thanks.