This is probably a common bird, as I've heard it off and on since I was a kid, but I have no idea what bird species it is.

I live in eastern Kansas and I've only heard it at night, usually in heavily wooded areas, but about 4 or 5 years ago I began hearing it in more suburban areas with larger trees. It appears to be a summer only migrant to the area as I usually begin hearing it sometime in late May or early June and it ends around the first week of September.

The "song" isn't any kind of a whistle or smooth song like say the song of a Cardinal or Robin or something like that, nor is it a hoot or shriek like an Owl species, nor is it a whip-poor-will or a chuck-will's-widow (we have both around here and it's nothing at all like their calls). It's a repetitive series of 4, what I can only call "acks" which seem to slightly descend in tone from the 1st to the 4th. They repeat this series of 4 "acks" nonstop. They seem to begin about 1 hour or 2 hours after sunset and end sometime between 2 or 3 hours before sunrise.

As I said before the bird appears to only call from heavily treed areas and they seem to prefer to call from near the very tops of the trees. Almost always 1 bird per tree and they seem to be somewhat spread apart from each other. I've tried to see one with a flashlight but failed to do so as they're so high up in the tree.

Again, I'm betting this is a common bird in the eastern USA.

Any ideas on what it is?