Cool DT! Certainly heard that peeping before. Wow if you listen a while to the link Munge provided, they make many different sounds. That engine sound is like the sound I couldn't explain that woke me more than a few times at the Lost World camping/parking area before. With the Yeti lore about the area, really great sound to hear middle of the night while all are sleeping.
One creature I love to see time and again around Soyo & the ridge is bear. So far seen two this season. One crossing over the road in front of us we had to put the breaks on. Another we turned a bend and it was running down the middle of the road we were on. Great views!
Another time a big mama charged up fast behind me on a tromp through the woods halfway to Fallen Eagle. I heard the loud huffing sound they make when objecting and shocked by human presence. I turned to see her, a golden wheat color, but she had turned to look behind her and ran perpendicular from the path down the creek to cross a giant log. Then we saw the two bear cubs following her across the log and she looking back to make sure they followed.
The ringtails John mentions, they were so funny - like watching a cartoon and the three stooges...