Why does a 2004 Impala LS engine make a clicking sound when cold that seems to disappear somewhat when warmed up?

It is probably the lifters. I would try some oil addative for sticking lifters and let it stay in there for a couple of weeks and see if that helps. low on oil? until the oil is warmed up enough to flow freely it could make that noise if low on oil. a 2004? i'd take it back to the dealer, that is to new to have ANY problems. good luck TAKE IT TO THE DEALER, IF IT'S A 3.4L AND THEY TELL YOU IT'S NORMAL COLD START PISTON SLAP, THAT'S WHAT IT IS. THEY ALL DO IT. IT'S NOT A BIG DEAL, OTHER THAN INTAKE GASKETS, IT'S A VERY DEPENDABLE ENGINE.