Well, they could be UFOs "breaking the density barrier" or they could be comet fragments exploding in the upper atmosphere. They could be sounds from space, too, I think.Speaking of John Keel and mysterious sounds from the sky, there are of course the "skyquakes" and the connection between possible overhead explosion and the UFO phenomena. Very good to hear that Keel isn't the only one talking about such things! Although it appears that he didn't consider skyquakes having to do with overhead meteor explosions. Thanks for the recommendation!
Here's the relevant quote from Keel's "Disneyland of the Gods".