I wouldn't immediately accept your mechanic's diagnosis. It's not common for the ceramic monolith to break up unless the casing has been bent or damaged. The monoliths are quite fragile out of the can, but they are mounted in matting which protects them from shock when fitted inside.
If you have serious engine faults, misfires etc, it's possible for the monolith to have thermal runaway and they can melt, but the engine management system will flag such faults and should usually protect the system by shutting off fuelling to non-working cylinders for example.
The ceramic isn't particularly hard and tends not to rattle, not such that in makes a noise you will hear anyway.
If it has broken up it would very probably give a fault warning (MIL). If it is driving normally then it hasn't blocked the gas flow.
My guess would be it's far more likely to be something metal, such as a heatshield as others have noted. Replacing a cat will provide more income that trying to fix a heatshield, not that I'm cynical at all.
Of course trying to diagnose at distance is difficult and it could be broken up.