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it would fit a sedan would it?
I would not think it has anything to do with carbon build up or lack of it , nor has it a lot to do with the fact that's it's an X-force either.
What i do think could be causing this is where the Tail shaft protector plate is located it has a very small tolerence between it and the muffler, so it could be touching there, and causing the vibs you are feeling.
That could be it, but i am not 100% certain.
Cheers.