I may be able to shed a bit of light on this one...
Check to see if the air cleaner assembly is vibrating against a bracket for some of the throttle linkage on top of the valve cover. When I bought my 85 300D, the air cleaner bracket was broken and the air cleaner did not vibrate much. Once I replaced the air cleaner bracket (easy except there is one really tricky bolt at the bottom) the air cleaner would vibrate and make quite a noise at idle. Revving the engine a bit (say, to 1000RPM) would make the noise go away until I let off the fuel and it returned to idle, whereupon it would make that awful noise again.
My mechanic (Precision Motorcars in Cincinnati - hello Aaron!) said that this was typical and that they had devised a "fix" for this situation. They recommended that you get an old air cleaner mount bushing, saw it in half, and attach it to the side of the air cleaner housing so that it acts as a cushion between the air cleaner housing and the throttle linkage mount. You have to drill a small hole in the air cleaner housing and bolt it on.
I haven't tried that yet, as my car has other issues at the moment, but I can't imagine a downside to this suggestion.
Hope this addresses your noise, or at least has helped someone else who faces the same situation as me.