I have a 2001 Z71 with a 5.3 with 102,000 miles. Lately I hear a high pitched whistling noise behind the dash coming from the engine. The pitch is consisent with the engine RPM...higher rpm higher pitch. Also sometimes I can turn off the engine and start it again and the noise is gone. When I pop the hood I cant hear it as good but I can tell it is coming from the rear of the engine. Any Ideas?