Exhaust issue - blown cat?

Ok, here's my problem.

Driving home from school today, I gave it a little hell merging onto the highway in 4th and took it steadilly up to around 100mph and let off. I got home, and then headed to the bank. As I'm sitting at the drive thru teller, I started hearing a rattle, right at where the driver side cat is located (I have a bassani catted X btw). If there is any loss in power, it is not noticed. I just had the car on a lift last weekend because I was getting an exhaust leak, and I tightened everything up nice and snug; it does not sound like an exhaust pipe rattle/loose connection.

I was sitting outside with the car running after I got off work about an hour ago, and the sound is not heard at idle, only when given some throttle. The best way to describe it is that it sounds like there is some type of metal meterial being blown around. Again, I have not noticed any loss in power, but the noise is very annoying and I'm supposed to have the car at the track on sunday and I wanted it running good - now this happens. It was going to be my first time at the track with this car too.

I'm going to try to get it into the shop on saturday at school, but I need some ideas of what to expect. My best guess is something is wrong with the cat, but I dont hear of many people blowing their cats out. I havent even run it WOT today, but I guess if the cats are bad they are bad. I also bought the X pipe used, and I do not know the previous owner, nor do I know what car it was on or what it had done to it. But I have had NO problems with it until now.

Any help is appreciated. Thanks in advance.

Thanks man I'll have to try that out. I still have the stocker but I really dont feel like putting that back on. It has band clamps right after the cats, so if indeed thats the problem, I may just pull the midpipe apart and gut the cats. Hopefully thats not the problem though because I do like the peace of mind of being legal.

UPDATE:

Now my problem is getting really bad. Coming home from school again today, it got to the point where I basically have NO power, the exhaust volume is noticably quieter, and at one point the CEL was flashing (severe misfire). It seems very clear to me that the cat is clogged up and/or blown, but is there anything else that could cause this? I'll be getting the car into the school shop tomorrow and I'll see what I can do. I'm hoping that if it is the cats, they wont give me a hard time about gutting them, because I really dont have a choice. I dont need a blown header gasket or worse, a burnt exhaust valve.

Any advice is appreciated.

This past saturday I replaced the driver front O2 which I did have a code for, however I didnt feel that I need to be in a huge hurry to replace it because even though the sensor was off, my fuel trims on both banks were fine. The O2 fixed my driveability problem, and for whatever reason I dont hear the rattle from the cat anymore. The CEL did come back on and I know the cat is broken up inside. School would not let me gut them, claimed it was "hazardous meterial". I have gutted cats before and yes it can be a PITA because its like beating rocks, but I wanted the sound anyway. School does not have a welder either, odd but they dont have one presently.