mine has done it since i bought it around 118,000. there are a lot of articles on this if you google it or even search the forum. it's piston slap due to gm changing the way their assembly line manufactures pistons to fit individual pistons. they used to match each piston to each cylinder, but eventually they just started mass producing pistons that were the exact same size....for some reason they don't all match up exact with the cylinders they are placed into because of this and a lot of our trucks get piston slap because of it. after enough oil gets to the top of the engine it stops the noise...gm says it's not a big deal.
That is exactly what GM says it is. I currently have 3 fullsize GM trucks 2 w/5.3's and 1 w/6.0 and they all do it. The colder it is outside the longer it takes to stop (the oil takes longer to circulate). I do use a lighter weight oil in the winter. GM says it isn't doing any harm, but it's hard to belive a metal to metal slap isn't wearing out the piston or the cylinder. Although I have gotten killer miles out of these motors. I had well over 200K on the last Tahoe I got rid of and it still ran great! I have noticed if you substitute a quart of oil for a quart of Lucas oil treatment it seems to be ALOT better. I think because Lucas adheres to the metal better than oil alone. I also have a tiny rear main leak in the 2500HD truck with the 6.0 and since using the Lucas I have gone from losing about a quart in between oil changes to less than 1/2 a quart. I am not a big fan of "mechanic in a bottle" but some products out there really do work well.
