I took off my air cleaner on the 300TD to adjust the valves. This
rubber thing was under the air cleaner housing. I cannot seem to
figure out where it goes.

My valves were way out. I thought it ran well before. Now it's superb.

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__________________
85 300TD FED-Daily
84 300SD-Wife's
86 XJS-Sunday
66 GMC-Work- Given to my stepson
83 BMW Airhead- Given to my stepson

That looks a lot like the rubber O-ring that connects your muffler and/or exhause pipe to the retaining hook on the bottom of the frame. There should be a couple at the back of the car and more near the muffler.

It's possible that the piece in question was placed (by a previous owner) between the air cleaner housing and valve cover to prevent the clatter that frequently occurs when the housing bangs into the valve cover.

I'd say both above answers are correct. Nice job guys!
It appears to be an aftermarket muffler hanger, and the reason stated above is its likely reason for being where it was.

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

That was my first thought that it looked like a muffler hanger.
I'm new to the MB diesel scene though and some of those rubber
things that go around some of the hoses look similar.

Leftover parts are always the bane of shadtree mechanics.
Then somebody throws in a herring. I thought ,damn, something
is gonna be rubbing on somethin' it's not supposed to if I leave this
part out. Thanks for the detective work folks.

__________________
85 300TD FED-Daily
84 300SD-Wife's
86 XJS-Sunday
66 GMC-Work- Given to my stepson
83 BMW Airhead- Given to my stepson