Under certain conditions, I have been able to make the sound using a Greg Hood single reed call, and a Howard Harland double reed call. But it seems to happen almost involuntarily. Once it starts, I can do it, but I can't just pick up my call at random and make that sound at the end of the note. It only seems to happen to that last 2 or three notes, at the end of hail call series, or something like that. (no acorn chuckles, please) The ducks didn't respond anymore favorably. And I think you're too prone to making an error at a critical moment. Long story short, I wouldn't know how to tell you what to do. It's all in how you push that last little piece of air through the call.
But if you call Haydel's, I know they'll try to help you. They helped me on the phone about 15 years ago. They're nice like that.
When you get it right, it does sounds pleasing. But I would recommend that you just reserve it to impress your friends who know, not the ducks. They don't care.
Hambone, I'm going to buy one of those ninja duck calls to go with my ninja shotgun.

