I recently took my new-to-me 1995 YJ with 4 cyl, 5 spd, and 115k to the mountains. As I climbed a long grade with the throttle buried, I noticed a new whistling noise. It sounded just like a turbo - it increased in pitch with throttle position, becoming more of a loud hiss at WOT, and changes in pitch lagged throttle input. The sound did not correlate with engine or vehicle speed. At the same time I felt power loss, which I thought at first was altitude, except except that while climbing another grade, with the throttle floored, the hiss suddenly stopped and I instantly felt a significant power gain. This happened twice in the mountains, and has not happened again in two weeks of driving around at sea level.
I checked codes today and got a 12 and 51. I believe 12 is less than 50 power cycles and 51 is Oxygen sensor - lean condition.
What could cause this intermittent whistle and power loss? Has anyone experienced something similar?