My 2001 truck was built with a 1999 engine that had been marked in a TSB for exchange. GM engines display a cold start knock located in the crankshaft main bearings. The knocking pounds the bearings to death. Eventually the engine will fail. GM is telling the dealers to tell the customer that this knocking is normal and that they built it into the engine([non-permissible content removed]). You can do an engine oil analysis on any knocking engine and it will show heavy wear(failure).