I spent 3 months in North Western Utah in 1999 doing graduate field work. I was alone for almost all of it. I read Lord of the Rings, played guitar, laid in the dirt staring up at the stars... very fun.
Anyhow, one night as I was sleeping, I heard a strange noise (see Edit below) around 3AM that jolted me awake instantly. I was in a tent and about 30 feet away from me, I heard a grunt that lasted about 4 seconds. Maybe it was a throat clearing? It was an animal noise.
Most alarmingly though, it had a definitely aggressive tone to it. It sounded like a challenge. There were no bears in the area - but mountain lions were possible, although quite rare. (I didn't hear any of the insane mating calls for the 3 months).
I sat bolt upright and grabbed my flashlight (I had no weapons with me other than a small knife and that was somewhere with the cook gear). I waited, quiet as a mouse for about 30 seconds listening for any noise. It was dead quiet. I could've heard anything. But - not a single sound.
As quietly as I could (but it sounded like when you're opening a bag of chips when you're trying not to make a sound), I positioned myself at the tent zipper then suddenly yanked it open and shined the flashlight. Nothing.
Then I bolted for the truck about 50 yards away. I made it and looked around with the flashlight. Nothing.
It was out there watching me - unless it ran away when I ran to the truck. I had the window cracked a bit and watched and listened for about an hour. Nothing.
I laid down in the back and tried to sleep but it was quite chilly and I was wearing only a pair of underwear and my huge scratchy wool sweater that was luckily in the backseat.
The next morning I looked for tracks but saw nothing. And for the next 6 or so weeks I slept in the back of the truck.
Edit: here is a similar noise that I heard. Listen to 0:05 to 0:09 (or more accurately from 0:40 to 0:45) of this video. I did NOT hear the super obvious cougar snarls at 0:10 to 0:13. https://www.youtube.com/watch?v=oVZCAQP6eQM