Anyone fold a private leaf in half clamp it between thumbs and blow - sounded like a duck being run over. Could also be done with a broad blade of grass but a bit more difficult.
Yes, though living in the Council house end of West Derby, I used privet rather than private leaves...
As you say, it sounded rather like a wonky Elmer Fudd duck attracter once you got it going, but it was a tad hit and miss.
I found a flattened 3" length of old bicycle inner tube could be made to work a treat though, a bit like the working farty noise part of a whoopy cushion as advertised in the tiny catalogue folded up inside a pack of Bazooka Joe bubble gum.
But despite best efforts, I could never do the thing with a blade of grass, I think I may even have cut my lip trying it and gave up after that.