Alright, so I bought my 02 with the 4.2 with 134,000 miles a few weeks ago, and noticed that in the mornings especially, my engine sounds like its knocking, only for it to go away after a few mins/miles of driving. I have a bunch of friends who have been auto mechanics for years, and my opinion on this "piston slap" condition is the exact same as theirs. Having experience myself going to aircraft school, and having learned about this, I believe I have somewhat of a decent explanation. This condition, in my opinion, is normal, as long as the sound disappears. A lot of piston powered aircraft can experience the same condition. They use "cam-ground" pistons, meaning the pistons are somewhat oval shaped at cold temperature, and the pistons will expand with heat, and provide a proper fit at normal operating temperature because of the high cylinder head temperatures encountered. This explains why the sound is prevalent when the engine is cold, and hardly noticed when warm. You'll hear different opinions on this, but I think the jury is still out. Let me know what you guys think. I know GM did this to save money, by not matching each piston to each cylinder, and this resulted in a sloppy fit. What's your opinion? Harmful or not?