There has been some transmission bel housing bolts come loose that will cause vibrations. I don't recall if it was at idle or at a particular road speed or condition. But checking to see if your nuts are tight shouldn't cost you much, unless you pay her

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1984 280SEL, 62,000 miles
Euro model in the USA
1983 300D (Totaled out 10/2004)
New Factory Mint Green paint
Palomino int
1982 240D (Steak dinner for two)