I have a 2000 ls1 in a 1972 Chevy C-10 the motor is all stock internals and has 160k+ miles....I havent started in prob about two or three weeks. Well i started and it as making all kinds of noise like rusted bolt being screwed in and out of hole then turned into a loose rocker arm sound...So pulled valve cover and didnt see anything but one rocker wasnt moving as much as others. So pulled intake and heads and found this on #3 cyl intake side....

So how does this happen?

 

Check the lifter tray that it came out of and see what number it is. Some of the trays marked "10" had a slight angle in them and were causing this type of problem.

 

Worn lifter tray, I fought this problem for a few months before I figured it out. Go to the Dealer and get some LS2 trays, they are a much better design, and fit just the same.