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I have an A/T RX8 with now over 20,000 miles. Like some of you I noticed driveline vibration, and lots of (apparent) rear-end noise, especially when the car was cold. I took the car into the dealer and they checked it out. One of the techs suggested testing the engine mounts. Sure enough, they were out of spec. They replaced the mounts and the excessive noise and vibration disappeared! The car runs better and smoother now, especially when cold. If you are experiencing any of these symptoms you might want to have your dealer check this out for you.
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I have also suggested that the engine mounts could be the source of excessive idle vibration. I'm glad that you shared your experience.
Do you know what they meant by "out-of-spec"? Engine mounts are designed to have a particular static stiffness and dynamic stiffness curve, as well as certain damping characteristics. Dealerships do not have the equipment necessary to characterize the mounts, so I'm wondering how they came to that conclusion. Did they send it somewhere for characterization? The only scenario I can think of is that there was a visible tear or some other type of damage to the mounts.
It is also possible that the NVH characteristics of the vehicle improved because the new mounts were installed in a neutral state. In other words, the original mounts could have been bound up (excessive lateral or fore/aft loading or twisting) due to a less-than-perfect assembly process. When mounts are bound up, their stiffness and damping characteristics are adversely affected, which can totally change the NVH performance of the vehicle. If this was the case, it could have been that the original mounts were fine, but just needed to be neutralized.