[Babar]

Agree with other comments that slow-fly prop is going too fast. I think it is either flexing and creating a resonance. Btw you should be able to see the flex by looking from the side of the prop, put a piece of white paper or ruler, as you accelerate you will see the tip move forward... NB most props flex at tips its a matter of degree.

Or, it could also be a prop-stall, where the airflow cant get around the prop, being a slow fly, wide at the tips and spinning so fast, it could be stalling... it's making the right sound for that, do you lose some thrust when you get the noise that would be an indication. Play with different props, you might want to try e-calc to help find the right prop size. https://www.ecalc.ch/

FYI- https://www.rcgroups.com/forums/showthre...Prop-Stall