My '85 500SEC has the same noise. I had it in to fix an emissions problem, and the tech said it was the track rod mount.
You can see the part at Performance Products web-site.
For my car it's under front suspension + steering; item #13.
The tech also said , that it takes two strong men and a boy to replace them.
A friend of mine suggested a silicon spray but it is too tight to penetrate. Just my .02 worth.
Good luck, John F