Weird apparent vocalization.
It was low and "gravelly" but extremely extremely loud. My recollection is it had a very slight "blue shift" then "red shift" in pitch. The first thing I mentally locked into was a log truck coming downhill on compression and rounding a corner, first coming towards me (blue shift) then going away (red shift). There was no shriek, no scream, no howl to it. The forest had been pretty noisy, birds, bugs, etc. It went dead silent but for the little creek by the trail, then after 3-5 seconds, the sound level built back up, but just for a second, everything stopped. In the moment, I thought I was "got". I was hiking a reasonably well traveled trail, traversing a steep slope, hill above and left, creek below and right, in a sort of clock-wise curve. I dropped / ducked to my left putting my left arm and shoulder into the bank, right knee to the ground, left knee up, and partially drew my pistol while scanning for "my attacker". As the sound rebuilt in the valley, I realized the source was a distance away, forward and left (NNW) and probably 3/4ths of a mile away.
Just over a mile away there's a canyon of sorts formed by 3 mountains (cinder cones?) in a slight arc. In the mouth of the canyon, to the east a quarter mile or so is a 4th cone. Between the 3 and the 1 is a deep notch. I assumed whatever I'd heard came out of that canyon and vocalized from near the notch even though they're all just a little farther away than I thought, maybe about 1-1/4 miles. I ran trail cams in that notch for 2 seasons. Got deer, elk, bear, cougar, a wolf, weasel ... just normal forest critters. Near the end of that period, I was hiking out cross country and stumbled across a raised rock outcropping. It was perched over the edge of the canyon. It's almost exactly 3/4ths of a mile from where I heard that vocalization. If you climb up on the outcropping, which is easy from the back (west) side, and go to the drop-off facing east, you're at the focal point of a huge natural megaphone above the trees. It's a perfect layout for truly long distance vocalization projection.
But ... what I heard wasn't a howl, it was a low gravelly sort of "ahhhh" roar.
2 months later I was hiking in the general area, this time with a recorder running. I heard, and recorded, seemingly the same thing apparently projected from a similar rock outcropping a couple miles away. This time the distances was about 2-1/4 miles across a fairly deep canyon with a river and waterfalls in between so the sound is a bit muted.
Those two are much alike and are totally unlike anything I've ever heard in nature. I've run trail cams on both hoping whatever it was will return. I had mechanical issues with the trail cams (5 years old, left out all winter ... no surprise) last year and removed them but I have new cameras to install as soon as the snow allows. At 6000 and 6400 feet respectively, it will be around beginning of July before I can access the sites.
The sounds were not good matches for any of the purported bigfoot vocalizations online. I truly don't know what it was. Oddly enough, I mentioned them to a friend from work that I bumped into at the rifle range. First thing he said was "gravelly". I have to talk to him again and find out how he connected the dots. Might know more than he is saying.
MIB