My Jeep makes a whistling noise in cool/cold weather (~60 degrees and below) that no mechanic has been able to diagnose let alone fix the problem. Here are the details...
-high pitched whistling (can be heard inside and outside the car - very loud)
-only when temps are about 60 degrees and below
-after 5 minutes of the car running
-only when RPMs are about 2.0 or below - however, RPMs can be higher when it is colder out and it will still make the noise
-as you push on the accelerator, the noise gradually decreases until RPMs are high enough to completely stop the noise
-When temps are around 50-60 degrees, sometimes the whistling will stop after the car has been at a complete stop for about 1-2 minutes
-If you turn off the car and let it sit for 1-2 minutes, then turn it back on, usually the whistling will be gone. It will need to sit in the off position for longer the colder the temps are
If you want more details, just ask. I'm desperate to figure out what in the world is wrong with my Jeep. This problem has been ongoing for about 4 years.