You have to keep in mind that the engine will shake just about EVERYTHING in that engine compartment. Since most of the noise comes from within a certain rev range, its most likely the frequency of the vibration and may not be the engine at all... but something else in the engine compartment.
I would have someone keep the throttle in a position that the vibration occurs and start snooping in the engine bay for the sound. (Just be EXTREMELY careful under there with the engine going, or you might become one of those 3 fingered high school shop teachers...

)
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Adam "Hammer" Packett:
Moderator, Gulf War Veteran
Holy Moses... Stop reading magazine times, put down the Fast and the Furious speed calculator, pull your head out... and read the da** TIME SLIP!"