We had a similar racket coming from a DC-3 prop.
The old boy with millions of hours in big round engines shouted ... he always shouted... (loud big round engines)... "Sounds like an exhaust gasket's blown".
He stopped her. I got to jump down and check the engine and found a peice of celophane paper from a lolly had been caught on the leading edge of the prop, with half over the front and half over back so it stayed there flapping.
Pretty loud for such a small thing but then have you blown a gum leaf whistle?