This trio gives a clear presentation of khoomi singing, from Mongolia. The strange, almost whistling sound comes from shaping the mouth like the resonating box of a musical instrument, producing overtones. One can control the overtones by altering the shape of the mouth, allowing a person to “play” an instrument.
I encountered this interesting style of singing as a teen. In my twenties I spent time to figure it out, but haven’t practised for a decade or so. It’s not hard to figure out, but as with anything, mastering it takes serious, concerted effort.