High Pitched Sound From Engine At High Temperatures

Ever since my 01 4.0L overheated back in August or September there has been a high pitched noise coming from the engine bay once it reaches operating temperature. It only happens at operating temp and goes away when I step on the gas. I've replaced the thermostat and that didn't solve my issue.

I'd look for a vacuum leak, or possibly a throttle body gasket leak. Engine produces highest vacuum when throttle is closed, and lowest vacuum at wide open throttle.

A length of pex pipe (5 ft for $2.00) can be used as a stethoscope. That should help you listen for where the sound is originating.