Whistling noise heard when some birds flap their wings--Paging Dr. Colibri

I’ve got a quick bird question so I hope our local ornithologist or another knowledgeable person will stop by soon.

I’ve noticed that when mourning doves fly there seems to be a whistling sound with each beat of the wings. My question is simple: Is this sound caused in some way by air passing through the feathers on the upswing, or is it made by the bird vocally, in the throat–as if the bird is “huffing and puffing” with the effort?

From this site:

Like mourning doves, a number of other birds have distinctive wing sounds, usually produced by modified feathers. The male Broad-winged Hummingbird makes a shrill whistle with his wings used in courtship and territorial displays using modified wing tips not found in the female. In the tropical manikins, males of many species have modified wing feathers that allow them to make very loud wing snaps and pops used in courtship. When you pass by a Golden-crowned Manakin display ground, it can sound like someone is setting off a string of tiny firecrackers.

And our very own Pennsylvania State Bird, the Ruffed Grouse, makes a booming thunder with its wings during mating season to attract females.

Sort of off-topic, but possibly tangentially interesting; I have a pet budgie that is allowed out of the cage to fly about a couple of rooms - I can always tell when he’s about to shed a flight feather because his wingbeats make a sort of shrill whirring sound, not unlike that of a pigeon taking off - presumably this is caused by some misalignment of the feather as it loosens before shedding.

Then there are birds that make very little/no sound with their wings. I was sitting in my yard one night at dusk. A great horned owl ( I assume-sucker was huge) flew by, scaring the bejesus outa me. At one point it was only 6 or so feet away yet was totally silent.

Indeed, making noise would be catastrophic for such a nocturnal hunter; many of its prey species (also nocturnal) are much more reliant on hearing than sight and would hear the owl coming, leaving it hungry.

Owls have specialized feathers on their wing´s leading edges; they have tiny barbs that act like turbulators, creating little air vortices that prevent the formation of larger ones and thus reducing the noise.

Which relates to a recent thread - I’m sure I can’t find it and if I did I don’t know how to link to it anyway - that discussed those features on airplane wings that have been designed over the years to interfere with the airflow in a way that ultimately causes smoother, quieter flight. As it turns out, it seems that the flight engineers have finally designed in something that Ma Nature had evolved into the owls long ago. Sooooo interesting. Ya gotta love this bbs.