you really need to describe the terms of the frequency and the variance of the sound in relation to RPM and load.

everything attached to the engine is vibrating; eventually, any loose vibrating item will cancel its own vibrations out (in simple terms) - use a 2x4 pushed against suspected items and use process of elimination. (unlikely)

more likely: There is a rotating item (like the fan!!) that is wobbling on its shaft trying to get free. Its vibrations also cancel eachother out in a rhythmic fashion.

least likely: One of the engine's balancing components have been installed improperly and the rhythmic noises you hear never cease but are ever changing w/ engine RPM and perhaps load.

just brainstorming

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