02 Jeep Grand Cherokee - High Pitched Whistling

So I noticed when it started getting cold that my Jeep emits this extremely high pitched whistling sound and I can’t figure out what’s going on. I’ve taken it to two mechanics and both said it was a belt that needed to be replaced so I had the belt replaced and the noise persisted. I’ve noticed a few things about the noise, it only happens when it is approximately 30 degrees outside or lower; only while the car is idling (when I accelerate the noise goes away immediately); It only happens after I’ve gone above approximately 30mph; The noise will go away after I’ve gone through a series of idling and accelerating; and stops immediately once I turn the car off. The Jeep is almost up to 150,000 miles.

If the mechanics could not hear the noise you may consider leaving it overnight on a cold night, or trying to figure out where it is coming from.

“only while the car is idling (when I accelerate the noise goes away immediately); It only happens after I’ve gone above approximately 30mph”

It can’t be both ways - only at idle AND only above 30mph. Are you saying that when you first start it up for the day, it doesn’t whistle at all? But then if you start driving and have gone over 30mph and then stop it THEN starts whistling but only when you sit at idle? But then it goes away when accelerate again until you stop and idle again? Confusing description.

Did the mechanics who replaced the belt actually hear it? Or just guess?

When a noise gets described as a “whistle” its usually from air movement - like from a vacuum leak. Belt related noises more often get described as “screech” or “chirp” or “whine” something like that. So could it be one of those instead?

FYI: if there is noise from the area of the accessory belt that doesn’t go away with a new belt, then the next thing to inspect is all of the pulleys and whatever devices those are attached to.

I’m not sure if this will help you or not, i have a 98 Cherokee Laredo, but i had a high pitch whistle like a boiling kettle coming from the engine from my grand am. We just changed the intake gaskets and could not pin point this noise. Finally after lots of coffee we pinned it to an un-properly sealed intake plenum, air was coming in from the gasket. It needed a bead of RTV to the gasket… noise gone. Check around the intake and check the Idle Air Valve. i might be off but could be worth a check.

Have you tried popping the hood and trying to discern from where the whistle is originating?
Mammals have two ears designed to isolate the direction from which noises come. Sometimes they’re the best tools you have.

Please realize that I’m not trying to be trite. I do care. But too often the source of a noise becomes obvious just by listening, and too few people do just that.