Re: 9.8B tohatsu prop issue. (whine with new prop)
Griff, I have been working on this issue for more than two years. Happens on some of the MFS8/9.8 motors, but not all of them. Tohatsu America has done extensive testing on this, and the factory folks in Japan are aware of it. What you are hearing is actually a sort of "whistle" that is occurring in the intake section. Think of blowing air across the top of a partially-consumed glass bottle of soft drink or beer. It is there (though possibly at lower volumes, so you may not notice it), whenever the motor is at about 2,000 RPM. The sound would not be heard at all if the motor was noisier, but since it's so quiet, you can hear the whine above the mechanical sounds of the motor.
When you change pitch, you change the cruising speed of the motor, so you are not running at the same RPM when at 1/4 throttle... hence, you don't hear the whine when you are pushing the 9-pitch prop... however, it is in fact still there, and when you throttle up a bit (to that magic whining RPM), you can start to hear it... but at that increased throttle, the motor is a little louder, and you don't notice the whine.
Since the sound is not detrimental in any way, but is only an odd characteristic of a few of the 8/9.8 motors, the "factory way" to eliminate the whine (at this time) is actually to change prop pitch (so you cruise at a different RPM), which is just what you have done... although you changed pitch in the wrong direction, causing you to cruise at that "magic" RPM, and notice the whine.
What kind of boat is it pushing? Maybe yet a different prop pitch will get you the performance you want, and allow you to cruise at an RPM without the whine.
