I've got a 1990 Cherokee pioneer with the 4.0L I6. Has a high pitch kinda whistle to it when the motor's warm. When I start it in the mornings to let it warm up, no problem, runs fine. But once it's warm, it's got a high pitch whistle to it. I can shut it off and restart it and it continues. Only happens when idling, when driving down the road-no problem. Anyone else having this kind of problem and might know how to fix it?

 

I had a Comanche that did that when I pulled the intake manifold off and didn't replace the gasket

 

You probably got a leak around the intake manifold/throttle body area. Try spraying some TB cleaner around there and see if it sucks it in anywhere, changing the idle. Then you will have found your leak