I stood in a park in my own city listening to one of these VERY LOUD strange sounds (for 20 minutes), that was no plane and should have stopped people dead in their tracks. As I stood on the hill looking over the park, a handful of people were walking their dogs and none reacted. I don't find this odd at all. I've had the largest military cargo plane go directly overhead in parking lots here and no one looks up. I don't care how often you see them, the noise and vibration should cause you to look up, but they don't. Most people seem numb to things around them. There is also the "normalcy bias" which plays a large role I believe. The same kind of normalcy bias where someone is standing eating a sandwich and doesn't flinch when a dozen police run by with guns drawn. I've seen it all, so I wouldn't judge these by others' reactions.