Ok here's a question for you guys.  I own a 2000 chevy 2500 4x4.  I bought it with 52,000 miles and it currently has 101,000.  Since I bought it I have noticed a ticking noise right away at engine start up that varies in the amount of time that it goes away.  The noise is more noticable at cold ambient temperatures.  To me it sounds like a valve train noise.  It has done this at 52,000 miles and has not seemed to worsen.  I use 5w30 oil and have taken oil samples that come back with normal metal conditions.  I have also heard many other newer series gm motors do this.  What the heck is it?  Is it the hydraulic roller lifters not being pumped up right away.  Like I said it does not do it all of the time.  Is there some kind of check valve that should keep the oil from draining out of the lifters?  I have even went to a 0w40 in the winter and a 15w40 in the summer because the pickup does get worked fairly hard sometimes.  I still notice the noise.  It also seems like the motor can be cold at startup and the noise won't be there if I previously started the motor within a short time frame.  I would hate to keep running the engine if it is a lack of lube noise.  That has to be hard on my lifters.  Thanks for the help.  

You have the classic 6.0 piston slap (pistons are too small for bores) There is a lot of them out there. It is not terminal and it will be worse in cold weather and it will never get better only worse with time. As engine heats up piston swell faster than block and reduces clearance so slap goes away or quieter. A heavier grade of oil can help (like 10w30 instead of 5w30 in cooler weather and 15w40 in summer heat) but it will get worse with time. GM has 10 of thousands of them out there do to poor production tolerances and now calls it normal. Long ago they used to fit pistons to bores on line (they would have sets of them in different sizes) but they stopped doing that before they came with that engine and they use one piston size for all and the result is that some slap. (when you use a new boring tool to bore block when tool is new it cuts a little extra oversize so that they can run it longer between changes as tool wears down. If you get one of theses block it can slap. Nothing you can do to fix it short of a tear down and use properly fit pistons or a rebore with proper pistons otherwise live with it sorry.