I firmly believe if it is speed-dependent, not RPM-dependent or not happening the whole time the motor is running, it's not the mounts. If it is while in gear, at speed, it is very likely the flex discs.

Ya know, Blackmercedes, what o you think about the tensioner spring ( ithink)? There is a piece that goes with the tensioner and I can;t recall exactly what it is. I've heard these cause shuddering. Minewas replaced under warranty, when I complained about engine rougness, but I don't personally remember it being that bad, but my seat of the pants is different than others

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL