It took me a while to learn how to do it, I caint roll my tounge, but if you can I think it is that except slow it down, if not I just said ticka, ticka, ticka, as fast as I could, and eventually it just faster and faster untill it sounded like a feeding call, now I can add voice, and break it up a bit, cause ducks don't make that sound for a long drawn out pieriod, its about as long as three quacks I guess, hang out around ducks and just listen, that realy helped for me.