Pontiac Repair: 2001 buick valve knock, roller lifters, noticible difference

Question
I have a 2001 Buick Century with a 3.4l engine.  It developed a knock a while back.  With the help of a neighbor, who is an auto tech, we replaced all of the roller lifters.  It didn't change the sound.  the car performs just as it used to, which is that there is only a noticible difference in performance when I punch it.  Then it hesitates a little.

Answer
Hi John when you say valve knock what do "you" mean? is it clattering like a cna of spray paint and ticking?? or does it "PING" when you drive it? there is a huge difference in problems one is a mechanical issue and one is a fuel or sprk delivery issue. if it "pings" did you try a tank of premium fuel in it to see if it goes away or it's better?