Diagnostic Aids
WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, 2003 Chevrolet Silverado, and 2003 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
If the vehicle is equipped with the 8.1L engine, the transmission is in neutral with the engine operating, and a whirling kind of a noise, neutral gear rattle, is heard outside the vehicle, this is a normal condition.