Changing one motor mount will raise that side of the engine, redistibuting more weight onto the other, presumably collapsed, remaining mount. You will thus couple vibration and noise more strongly through that mount. You will also change the inherent torque axis of the tranny output into misalignment with the propeller shaft, and could notice some problems there.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7