Question--

I have an 02 silverado 4.8L. About 300 miles ago (5 days ago) I started noticing a slight tapping noise that I thought was a large heat shield that was coming loose or I was getting detination into the exhaust system. the sound seemed to definatly be coming from some part of the exhaust and was lower sounding than detination (pinging). I thought I had gotten ahold of some bad gas and put in fuel additives to band aid the problem. That seemed to help some..maybe....and I have refueled since. Now with 93 oct gas I'm getting the same problem...and now its a little louder. This afternoon the check engine light came on and Autozone pulled the code and quickly reset it before I could ask what number it was, the guy forgot. It said there was a converter effeciency problem, basically the rear O2 sensor was misreading. I got to thinking then that maybe my cat(s) are breaking up and the noise is the honeycomb moving around in the case.

Any Ideas??? Thanks....Oh and the sound only happens at just off idle or when the motor is under a load, crusing is quite.