If you can hear a noise from inside the cat., that would not be normal so is likely your problem. It will often be heard more during acceleration or deceleration.
If it is starting to crumble, what happens first is the packing that holds the pack in place comes apart. That give the pack holding the catalyst the opportunity to move around - thus the rattle. As the packing deteriorates more - the rattle gets louder. Replacing the cat is the best solution.
It'* not real likely that it was caused by the crash.