By law you are not allowed to mod your cat, but I know how it sucks to have the damn thing rattling all the time, mine kept doing it. This is what I did, but it was only a temp solution, I removed my cat and took a 60 LB Jack hammer to the inside and cleaned it out nicely. The effects of this are that, it makes for a kinda funky hollow cat sound on your exhaust. The O2 sensor is an after cat sensor that has no effect on the way your engine runs. When I hollowed out mine on my 98 it took about 3 weeks for the check engine light to come on, when that happened all that I had to do was unplug the sensor for a couple of days until, the computer forgot about and would trip the code saying that the O2 sensor was unplugged, at that time I would plug it back in and the light would go off, I had to do that 2 time, and then it stayed off for almost a year. Although I can't recomend canning your cat because it is illegal, I can say that I live in MA which has an emissions test similar to california, and my Jeep passed with flying colors. The best solution would be to purchase and aftermarket cat and have the O2 sensor Bung installed so then you would be legal and not have the annoying rattling noise.
