Okay so as of right now my SES light is on and the code is the PO430. I know that it can either be a cat problem, an o2 sensor problem or a vaccum leak problem. Firestone says its not a cat problem but what do they know anyway. My car is due for an emissions check by may so I kinda need to fix this problem as soon as possible. So the other day I had the hood popped and the engine on, I went to rev it (thumb on throttle body so i could hear anything different) and i noticed that as I rev it the engine emits a whistling sound. Now I dont have a CAI or any type of intake (Stock airbox with K&N). I was wondering if this could perhaps be a vaccum leak that is the source of my SES light. Any help will be greatly appreciated.