Thank you for posting the video -- very helpful
I have no experience of my ducks making a noise like that. However, I notice the movement accompanying the strange sound. Two of my pekin drakes make that movement and come out with a belching sound. One drake is 18 months old and one is more like 12 months old [He was rescued at the beginning of April and was a juvenile then].
When my first drake started making the sound, people on this thread suggested he was adjusting his crop. It got to the stage when I firmly beleived he just liked makign the noise [and he still does.] A second pekin starte a couple of months ago. not as often as the first but then a year ago the first was only doign it occasionally. my third pekin and my son's pekin drake do not make the noise and neither do my muscovy drakes. The females don't do it either.
As your boys are otherwise healthy and eating normally, I would suspect that for whatever reason they started adjusting their crop and then found they like the noise. Or maybe they just like adjusting their crop. Your boys' whistle may be because they are call ducks wherreas mine are pekins that belch!!
Certainly, while the ducks are healthy and eating normally, I would just wait and watch, I can't think of an intervention that would help