What's Causing the Weird Trombone Noises in the Sky?

There's examples on this article on the Baltimore Sun from let's see New Jersey, Texas, New Zealand, Chicago, Australia, all over the place. And it is, you know, there is some similarity to the sound. It does have this kind of weird supernatural, ethereal quality to it. Some folks would like to attribute this to HARP, which is the US government's high frequency, active oral research program that's analyzed the ionosphere. The less conspiratorial explanations for this would be things ranging from power lines to small earthquakes and electromagnetic radiation.