1998 Moomba Kanga intermittent noise

I've got a 1998 Moomba Kanga with 75 hours on it and trying to locate the source of an intermittent noise that I'm having. The boat has the 310 horse Indmar and ZF Hurth tranny /hydraulic unit. Its sounds like a high pitch squealing noise. Almost like a belt slipping but worse. The first time it occurred I couldn’t pinpoint where the noise was coming from. It sounded like it was coming from the front of the engine so I replaced the impeller on the raw water pump. Next trip, same noise but I had some other people with me who could raise the cover and look closer to pinpoint the noise. It seems like the noise will not occur if I'm running along at lower speeds. Once I plane out like to travel to a different part of the lake and run for a while, the noise will start. Once it starts, you can see the vent cap on the transmission vibrating and the noise seems to be coming from the transmission area. There seems to be no slipping of the transmission when this occurs. Transmission fluid level and temp all look good. The vibration that accompanies the noise is not that bad. If it were not for the noise, the vibration is so minimal that I wouldn’t even give it any consideration. When running the boat planed out and the noise starts, if I throttle way down the noise stops. If I throttle back slightly the noise continues just to a lesser extent. If I slow down and let the noise stop and then get right back on plane, the noise will return. It seems like if I slow down and wait for a while, the noise takes a while to return once I get back on plane. I thought maybe it was cavitation but it seems like the noise would go right away once I dropped the engine speed some if this were the case. I have looked close at the shaft. To the naked eye it looks good and straight. The rudder ??? that the shaft goes through and it holds the shaft at the bottom of the boat looks straight and no apparent signs of hitting anything. The rudder is solid and wont move if you put pressure on it. The bushing that supports the shaft through the rudder (cutlass) shows no slack side to side or up and down. Again the boat has very low hours for its age but that really means nothing.... The prop is dinged up some and one blade looks a little bent at the end. This is on my to fix list. I dont know if the prop would set up this noise or not.. I decided to try to get the sound to re-occur while on the trailer. Realizing that there would be no load on the shaft, transmission as you would normally see while on the water, I wanted to try anyways. I set up the water to the engine, water hose to run on the rudder to keep the Cutalss (?) bushing / bearing cool, one hose to spray on the shaft nut / packing from the bottom of the boat. Lastly one more hose to run on the shaft nut packing from inside the boat... Cranked the boat up and ran it at higher RPM's for more than 1/2 hour and just as I figured it was flawless ...no noise. I'm not sure who to try to call for advice. I’m down on the Gulf Coast in Texas and its hours to the closest service. I’m sure this kind of diagnosis can get pretty pricey. If anyone has any suggestions please reply... Thanks, Cliff