Looks like cicadas will be making a comeback this year according to this article. (That might be what I've been occasionally hearing at night as well.)
What's that noise? The 17-year cicadas are back
With warm daytime weather and mild nights upon us, you may find yourself opening a window to enjoy the cool spring air. But accompanying this breeze will be a cacophonous whining like a field of out-of-tune car radios.
phys.org