My ex-wife had the same noise with her 2000 TA. When taken to Pontiac they checked it out and they said that it was just noisy valves and that it was "normal" with those cars. I was not really thrilled with that answer and took it to another dealership since it was under warranty and they said the exact same thing. I ran a better oil in it and it seemed to help the issue a little bit. Try a little thicker good quality synthetic in the summer months. Other option is an exhaust leak. It can sound like a ticking noise as well. As for the cats...well if you get some good engine management you can take em off