After driving about a mile, the jeep is making a high-pitched whistle from under the hood. It makes the noise when my foot is off the gas pedal. If I am on the gas, even slightly, there is no noise at all. If I sit and idle in park for about 5 minutes, the sound slowly goes away.
A few things come to mind when you describe this noise. Do you have a manual or automatic transmission? If you have a manual you could be describing a throw out bearing in your clutch. If you are experiencing any performance issues, it could be a restricted catalyst converter. On your model, I have even seen the PCV valve cause a noise like this. My best suggestion would be to have a mechanic, such as one from YourMechanic, come and inspect the whistle noise that is coming from your hood.