Hey Oklahoma peeps!

Can you help me with a sound?

There are things in the trees that make loud noises that I can only describe as the sound a very large rattle-snake would make when it rattles a warning. They don't sound like any locusts or cicadas I've ever heard and I've not seen any, so I highly doubt it's either.

What kind of bug would do this? I've never heard of a bird doing it, so I assume it's bugs. One tree will light up with the sound, then the next tree across lights up with it and down the line... it appears they're up at the tops of the highest trees, not in any of the smaller ones or in any bushes.

Got any ideas on what's entertaining me so much? I either didn't notice (unlikely) before or they just started a couple weeks ago.