For what it's worth...
 

This is just something that happened, was sorta weird so I thought I would share incase someone else run's into this ( or something like it).
I have had this rapping sound in my engine for months, I went over it with a stethoscope, had a couple of "old boy" mechanics look at her, even took it the GM dealer to a *LS1* certified technician..
Replaced timing chain (went w/ Cloyes Hex Adjust for later), pulled push rods, rolled em on some glass, inspected every rocker. NADA.. still rapping.
Got worried at this point..Then my SES light came on..PO430.. I read the forums, tried the swap the O2 sensor out, bad cat, blast, found out they were covered Dwane Laynes in Arlington was nice enough to diagnose, order and replace it with NO hassle whatsoever, they even washed my baby twice,
..Now the point of this. On the way home I suddenly realized ... No More Rapping....
Durrr what.. ? I can only conclude that the honeycomb in the cat was loose and rattling, the sound was then transferred via the header pipe and resonated at the engine ? Is that even possible ... ?
I remember thinking at one point that the rapping / tapping was louder at the door and behind the car, which also freaked me out, keep in mind I am NO mechanic, I can change oil/ starters etc and did my own stereo...I am embarrassed to say that a buddy helped me pull rockers etc..any thoughts
anyone ?? :bang:

This could have happened to anyone...this is by no means an isolated incident, as I have heard of other people having the same problem (usually with aftermarket cats though). Glad you got it figured out...