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He's got another thread that isolates the sound to the powerhead.

WaCo, here's a thought. It could be that the stator is too close to the flywheel and is rubbing. I'm not sure what a new 4-stroke Merc looks like under there, but someone here will have a clue.

You could use a stethoscope (or a piece of garden hose) to try to pinpoint the source of the sound. HTH