A friend of mine has a "gofast" boat. 454 w/exposed headers, Berkeley jet drive. At 2800 rpm, in the water only,the boat makes a sound like someone sticking a screwdriver in a fan. The engine has been pulled by a professional, no fix. Ideas?
We'd guess its a problem with the drive, not the engine. Check the bowl bearings and the bowl vanes.
Also, you might want to consider finding another "professional".