I would also check your exhaust to trans mounting bushings if your car has these (my W124 does). After you replace the motor mounts, thereby damping the engine vibrations, all sorts of other noises become more apparent. The noise you describe could be these bushings, or, as G-Benz states, it could be the tranny mount.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280