its carbon build up on top of pistons. only makes noise on cold start then goes away as truck warms up and carbon get soft. gm had a big hoopla over this and was gonna recall trucks and short block the engine. but then realized how many of these vehicles were out there and decided to put out a bulletin explaining the noise. my yukon has been making noise since about 40k and now at 100k same noise but no problems. i would not worry about it. a company called BG products makes a fuel cleaning kit that clears it away for about a month then it comes right back. tried every product out there on my truck from every distributor i buy product from none gave it a permanent fix but the BG product worked the best.
i have a 2000 tahoe 68000 miles on it n everytime you start it say next morning or hours later it ticks like a lifter tick dont know why it has the 5.3 it goes away but it just bothers me has excellent oil pressure when its started n running runs great just dont know about the lifter tick or what ever it is. was gona take to dealer. i wouldnt think it would need a cam n lifters but im not a certifid mechanic thanks for any help.
