The tone that persists in you recording is more than likely reverb from a hill or some other structure that the sound can bounce off of.
Yeah, I initially had the same thought. But if this truly is an echo that I'm picking up, then I would expect the same phenomenon to show up during the spool-down (which didn't happen). But who knows...this siren is quite faint by time it gets to my house, so perhaps I'm too optimistic about my ability to analyze it properly. At any rate, thank you for weighing in on this one. I'll be happy to finally locate this thing, and then the mystery will be solved once and for all.
2T22 or not 2T22...THAT is the question.