Western Rockies, 6000 feet elevation.
Time of day
This thing only shows up at nightfall. It is gone by early morning even when it is still pitch black outside.
It sounds like an adult male human saying "Hrmmmmm" as if it were displeased with my presence.
Am I hearing things
A family member heard this as well. I wasn't actually going to tell them because it would have spooked them. They told me after I had already heard it twice. We hear it every night at nightfall. I am going to try to record the audio.
It is always behind me at my level. I turn around, wearing a powerful headlamp and can not see it. I also have a 100k lumen LED flashlight. I can not find it. The sound does not move position and I can not hear wings.
What might it eat?
There are moths, lots of them. I have been getting rid of the moths to test that theory. There are also field mice, grasshoppers, crickets, grass snakes and marmots. Most of the marmots have been in their holes and they stay very clear of humans.
None. I have been in the wilderness all throughout the U.S. I have never heard an animal capable of making this sound. I have heard MockingJays and Starlings mimic other creatures, but they can't make the bass/baritone sounds like a human. There are horses near by, but they can't make this sound, certainly not in between the house and my vehicle. The mountain lions, bears and elk don't go anywhere near my home.
I should add that it also knows which way I am facing. It will only make that sound behind me, never beside or in front of me. It also knows if my headlamp is on. I have to turn off my headlamp to hear the sound.