Having a weird issue that has been happening more often on my 97 4.0L. When I step off the gas on the highway, it will make this high pitched whining noise. Doesn't really seem to impact anything besides my ears.
I got a video of it here.
Usually it will go away as I break, but on occasion, It's stayed around even when the car is in park. On these times, I open the hood and try to find the source, but no luck yet, as it usually stops pretty fast.
I just replaced all my vacuum lines because I thought that may be it. Replaced all the boots too. Maybe I missed one? I did PVC valves and all the hard black oem lines. Also the brake booster. Is there a spot I could be missing?
My only other guess is possibly a clutch fan bearing, but because the noise changes in pitch a little bit, I find it hard to be that.

 

Sounds more like a vacuum leak (although I couldn't hear it in the video). Check your intake manifold and throttle body bolts.

Next time with your head under the hood, grab the throttle linkage and gas it a bit. Take the rpms to about 3000-ish and let go of the throttle. The high vacuum should produce the whine (whistle).