What's sign are catalytic converters going bad on this car?

I had to replace my converters on my 2004 C240 recently. Thankfully under warranty at the time. I head a rattling sound from directly under the car when reving the engine. That was the lining of the converters breaking down and rattling. At one point, the ceramic lining broke off and plugged the exhaust causing the car to go into "limp home mode". Once it sat overnight, it started fine and could drive it to the dealer for replacement.