Today I spoke to a friend from church who just retired from being a game warden. He said it sounds like it was a human and he's never heard anything other than a human whistle that way.
There was a neighbor who threatened last winter to hunt on our land and the adjoining neighbor's land without permission this year, something about a 10 point he saw, but he's in prison for rape and incest so he's not a suspect. There is a 20 year old son of the neighbor all the way at the back of our property who asked permission three years ago and I said no and told him about being shot at twice. They have plenty of land to hunt on anyway. So, other than him and his buddy, I don't know, but will figure it out. Next time I might fire a warning shot and see what runs and hope they end up on the camera(s) we moved around Saturday.
On an encouraging note, there's a 10 point 160 yards from the house right now tending a doe. Yep, we saw him at 4:16 tonight and literally just heard her bleating. So, I'm going to go out in the morning and try again.