I have a 2000 Silverado Z71 with the 5.3l engine. In the morning when I first start it up I get a very loud knocking noise in the engine for about 30-60 seconds. I thought it was a valve problem. I took it to my local Chevy dealer and they said the noise is called "piston slap" where the skirt of the piston is hitting the cylinder wall until it expands enough. They said that they have contacted Chevy about this and were told it is normal. As far as I can see, this noise is not normal. No engine should make this type of noise for any period of time. It sounds like the engine is tearing itself up. How can that be "normal". Has anybody else experienced this problem? Is it a problem? My warranty runs out soon and I'd like to get it cleared up before thast time, but I need ammunition to push them into a repair. Any info would be greatly appreciated.