Man, you are a funny guy))) I check your posts and know that I will crack up... what has O2 to do with a sound? you have your way of examening stuff, that you actually never check after all ))
Exhaust manifold gaskets are hard to replace, not only lifting the car is neccesry, but have to remove most of the stuff from engine bay to get to it.
BTW you can hear it working from 3000-6000 because your engine works at that RMP.
Do you hear any weird sounds when it changes gears?