Please don't overthink it. It is really very simple. Blow a fast airstream at the palm of your hand. Now do it while you are humming a pitch. Vvvvv is the closest way I can think of to describe the sound. Once the lips close it goes instantly to hmmmmm. When the lips open it goes back to the Vvvvvv sound. What your vocal cords are doing is identical in each instance.
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