I see several of these complaints every month. Most of the time the noisy lifter(s) can be easily located. They should replace them all, though. Ask them to inspect the camshafts carefully. If they find evidence of "chatter" marks they should replace the camshafts, rocker arms, and lifters. None of this should be a surprise to your Service Dept. I've also found carbon fouling of the pistons to cause complaints of engine noise, usually following a cold-start. This noise can be mistaken for lifters to the untrained ear. The way I repair this is simple. While the valve covers are off to inspect the valve train, I remove one spark plug from each cylinder and pour GM Top Engine Cleaner directly onto the pistons. The longer you let it sit the better, preferably overnight. Before the plugs are re-installed I crank the engine over to remove the cleaner. After the initial road test I change the oil and filter. If you don't want to use the GM cleaner, Techron works well also, just not as quickly. I hope this helps.