My exhaust was rattling after I changed the Y-pipe awhile back - I thought it was a loose place where I screwed it on. But instead, it was a plate on top of the catalytic converter, which was mostly deteriorated. So, I used a screw driver and popped it off :P so now no more rattling.. but I think it was a heat shield, for when I took off my back seats yesterday, some of the foam was a little dark - could have been there before though. I drove around for awhile, but area didn't heat up, so I don't think it was a sudden thing. I could see through the cat, but should I change it?

And would there be a performance gain from replacing it with a hi flow? Any place to get it?

Why not drill some holes and attach the HS back with some screws? I'd weld it but I doubt you have one. Or take it to an exhaust shop and ask them to do it.

pick up a used cat off a B14 or B15 sentra (or pretty much any Nissan that's newer than a 3rd gen Maxima). they use cats with 2.5" flanges on it. much more flow, still get OEM quality.