I worked on an early 450SL that had the same problem. It turned out to be rock hard motor mounts.

If you leave the car in gear at idle, open the hood, and listen carefully, you should be able to tell if the shaking is coming from a rough idle. Of course, be sure to secure the car so it won't move.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe