harmonics

My 1973 Ericson 29 A4 emits a high pitched squeel or whine at the mid RPM in forward gear (usually somewhere between 800 and 1400 RPMs) The previous owner says this is simple harmonics and there is nothing to be done but put up with it. Wonder if anyone else has this issue and if you agree...

Hey All, I have an E35 MKIII Hull#211 M25-21hp Universal. Right now I get an odd howling noise at 2000 RPM and above. I am told it's a prop issue. (Prop Harmonics) Anyone familiar with this? The current prop is a 3 blade 15x11RH. Another interesting note is that the motor is rated for approx...