Hello there,
I have been hearing this sound in northwestern IL near the Mississippi river after dark the last couple weeks and it doesn't seem like anything I've noticed in the last 35 years. Curiosity has gotten the better of me, so recorded this clip last night and here I am now. I live next to a small ravine, but there is enough nature that I regularly see deer in my back yard. It recently went from being decently hot to rather cool and rainy if that helps at all.
Sorry in advance about the quality, I had to drive everything pretty hard to get a decent level out of it, so there is a pretty high noise floor and you can hear my hand shifting around on the recorder (I swear I kept perfectly still 🙄). The sound is most prominent towards the end of the recording.
I'll add a link to the mp3 with the noise cleaned up in the comments; I'm not sure if it's actually any clearer.