Yesterday I was driving and noticed the Check Engine light went one, so I ran over to autozone and the code is p0420. I have no clue what the help could be wrong. Says the cat could be bad but it doesnt smell like eggs or anything. Had one of my buddies look at it and he says the cat is still working. Also it could be a large vaccum leak. But my vac seems fine because the boost gauge says it is sucking 21 pounds of vac. I am though hearing a hissing noise coming from somewhere around the throttle body. I recently put a throttle body spacer in but i made sure it was tightened all the way down. I check twice on that but I am still hearing the hissing noise and we have no clue what the hell could be hissing any ideas anyone?