Generally a bad cat is one that pees on your clothes, sprays or bites you for no reason. Rattling, aka purring, generally means the cat is not being bad.
It very well might only rattle at certain frequencies. The same harmonics that cause enough 'turbulance' to make a broken cat matrix rattle can also be what makes a cat hit a heat shield though.
I always have to get under the car do some testing with my fist and a prybar.
are emissions an issue? cut them out if u can! u can even sell the cat guts for some $$$$. theres platinum in there!.... my old shop used to get our converters stolen from a locked bin all the time for that reason....
when the exhause is cold if you pounded it with your fist and hear anything that would probably be a clue if its all broken up. if you can unbolt it and look through it thats a bonus
do they break up just from driving? if so, why/how? i used to gut the things all the time in a shop, and that sh** wouldnt come out easy! it took 30 min with an air chisel sometimes just to break it all up!
The spot welds in the matrix break off. Then the entire matrix (honeycomb) rattles around. So it makes noise but if the honeycomb is still intact itself (or even broken into a couple of pieces) it will not fall out. It just clogs the exhaust up until the backpressure heats the engine compartment to the point of things starting to melt (or the interior catches on fire).
I believe I have a catalytic converter going bad as well, I had a '99 v6 several years back and I hauntingly remember the sound...It wasn't so loud at 1st, but it got to the point where it was SOOO loud that I was turning heads when I didn't want too ^^
It looks like my '03 gt is starting to do it as well...
Turn up the volume , around the 14-11 sec mark of the video you can hear it well, usually happens around the 1500-2000 rpm range... gear doesn't matter nor if I'm moving or not.