Sheri, you're not kidding!
During my up close - accidental - encounter, this thing was (I hate to call it running, exactly) coming mighty fast in more of a crossing of the mountain slope, and I never heard anything. I was getting growled at real loud by what I assume was another one from the nearby cluster of trees, but for something that big to cover that much ground, and never make a sound - in reflection - was quite a feat.
A week or so later, the team member that I had dropped off when all this happened, found another "sweet spot" with bars - and he went up the first switchback to again, call home. He came fogging it back down mighty quick, as he'd seen another. This was a bit different. There was an open area much like a meadow, and he said as he was talking with his back to the meadow, he heard heavy "thumps" and spun around to see what it was.
He was shocked to see this thing shooting uphill in the meadow. He said, "You remember that scene in Jurassic Park, where the puddle of water showed the vibrations, and then the deep thumps as the T. Rex approached? That was what it was like."
He wondered where it came from, as this was open ground, with only grasses and mountain flowers in the meadow. I told him that as he approached on the ATV, the thing flattened out in the field. Only after he turned his back on it, did this critter feel it was time to clear out of there. The flowers and grasses were not even knee-tall. And he wasn't looking for anything in the flowers and grasses - just a casual visual scan.
Combine that ability with their natural "ghillie suit," and folks are just not going to see them. Most everyone's looking for an upright critter. No telling how many times I've been out in the wild and had one real close and never even thought about one - and it remained undetected.
If you're out looking for six-foot, blue tinted, green-eyed men who wear turpentine-soaked rags around their ankles, shave their heads, and wear purple hats - if you don't see these blue tinted, green-eyed men with turpentine rags, shaved heads with purple hats - that you're expecting - you won't see them.
They seem to make noise only when they want to - from everyone I've talked to, and from the hundreds and hundreds of narratives I've read.
Good observation!