If someone were to perform a search, as I have, they would find a ton of kaka on this very topic. I have an 06 4.8is (CPO) with ~70K and she had the same problem. I took it to my favorite stealer and was told that since I had aftermarket cross-drilled rotors and Red Stuff pads that that was was the culprit. What a bunch of crap! So, I purchased a really cheap pair of cross-drilled rotors for the front and the problem continued.

But recently, I took it to another shop that I still trust and they found that the "fluid-filled" control arm bushings were leaking. They were replaced and 85% of the vibration is gone. As an electrical engineer, that's almost within the usual 10% spec...

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///BRUCE