If you can't see anything on the fan, you could still test it by removing the fan belt for testing and see if you still have the vibration. Just don't run it for a long time. If vibration is gone, it would most likely be the fan, or a loose drive pulley.
Just my 2 cents. By your description, it is nothing in the drive train, because it does it in neutral.
Everet <img src="/forums/images/graemlins/patriot.gif" alt="" /> <img src="/forums/images/graemlins/cheers.gif" alt="" />

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