Ooh... ooh... this just happened to me and I don't believe I've forgotten already. Check the bushings in the little shock absorber near the coolant thermostat. The bushings of that little shock in my 91 300SE were shot and it made a heck of a racket. You can check it by applying sidewards pressure on the shock body and listening for the sound to change or go away.
83 300SD ... $950, see cars forum
91 300SE ... sold