I've lived for years about eight miles NW of Boston's Logan Airport. Although I can't say we get a lot of aircraft noise, once in a while, due to winds or air traffic, there will be a long line of planes traveling right over my house, perhaps 6000-8000 feet up, descending into Logan. At night you can sometimes see three in a row, with a fourth one turning on its landing lights.
Most of the time the overhead planes sound mostly like a roar with some jet whine. However, once in a while a plane will suddenly start making a very strange sound like an enormous blown Coke bottle. Often the pitch will descend during a half dozen seconds, stay constant for a dozen seconds, and then rise again until the sound disappears.
My best guess is that the sound is caused by the pilot lowering the wheels during their descent, and that it only happens on certain models of aircraft. As the wheel well covers open, the air blowing across the wells sounds like that giant Coke bottle, with the pitch decreasing as the opening increases. On certain models, the covers will then close once the wheels are down, causing the pitch to go back up.
(Edit: it's been pointed out that gear isn't dropped until 2000-3000 feet, so in my case that isn't the sound. So, I've removed all those videos, cool though they may be.)
Is my guess right? What model/manufacturer of large jet would make this sound?