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Sounds like what you have is a resonance - the prop rotation is hitting a frequency that's causing something else to ring like a bell. To fix it, you'll need to either change the freqency of the prop rotation, or find out what's "ringing" and damp it.

When my wife complains about something rattling in the car, I first tell her to clean all the crap out of it, so that's my first advice - remove all loose items from the boat and see if it helps. If not, then try some damping - if you have it localized to an area of the hull, try stuffing some cushions or life jackets up against it and see if that helps. Or duct-taping a couple of old magazines may help if it seems to be a hull panel that is vibrating. Once you have found what's ringing, you could try a permanent fix some sound dampening mat like that used by car stereo installers.

Good luck! Sounds frustrating - you'd hate to have to mess up the performance with trim tab or other adjustments just to get rid of the noise. Our Aquasport has had a resonance since new, it's only just above idle so it's easy to avoid.

Tim