Mysterious Sound in game - Anyone know what this is??

It's definitely a creepy sound. If we didn't already have aliens in our game, I would attribute it to an alien invasion about to happen! Can't be StrangerVille, as I don't recall such sounds when I played through that scenario very recently. At first I thought it was a jet, but if we don't have cars, why would they put airplanes of any sort in the game?

I do hope someone has an answer. That would freak me out if it started up in my game.