Resonance coming from an outboard ???

I'm actually asking about this for both my buddies boat, a 17 1/2 foot fiberglass Kenner, with a 75hp 4 stroke Merc, and my new boat, a 14 1/2 ft aluminum with 25hp 4 stroke Yamaha.

Now my motor is basically brand new, and runs great, but his has very low hours, and runs perfectly also.

However, coincidentally, we both are having the same problem, and in both of our cases, it was after getting a new prop. Ironically, we both feel like we have finally got just the right prop, for the correct RPM's, and peak performance.

So here's the deal..... When I throttle up to about 2000 to 2300 rpm's, I start getting a whining sound, reminiscent of a tuning fork (except its off key.... so its hard to sing along with :-) I'm not sure what rpm's my buddies starts doing this. But either way, as soon as the rpm's climb above this range, it goes away...... but it does the same thing again as we are slowing down, and the rpm's pass through this range again.

I'm just about positive that this is not a mechanical problem, but on the other hand, it is annoying, and I have no idea what could be resonating like this, in either mine, or my buddies motors ???

Anybody have any experience with this type of thing, or ideas to eliminate it ?

Thanks in advance.

Peace,

Fish