I read in Motor Trends There've been alot of complaints about cold-start knock in the Vortec engine family, ang most General Motor's customers are getting incomplete answers at dealerships. We've spoken with Matt Kester, assistant manager with GM Powertrain Product Communications, about the situation. He said GM engineers are aware of the problem and are currently in a reaserch stage. The cold-start knock involves the 4.8L, 5.3L, and 6.0L V-8s in all applicable GM & trucks from '99 to '02. They believe the cause is carbon buildup on the circumference of the piston above the top piston ring. This interference is affecting the dynamics of piston travel and therfore causing a knock. I found this interesting