Boy, that's a good one! You've already done/ eliminated everything I can think of except...
I'm thinking excessive spark advance, even
Sounds like you have a "spun" prop hub; put a mark with a sharpie on hub and prop then run it till you feel the slip. If the marks don't line
Walk..........RUN away............it will never work........by the time your done you will be done with boating..........
Purchase a boat
I was on my way off the lake the other nite and was pulling a buddies boat behind me to the landing we were almost to the landing and I let the rope go
OK. Was at the boat last night. All 8 are running. Boat idles at about 600 rpm. Put the boat in gear at idle and I get the noise. Rev it to about 700