The cataylic converter helps reduce the emissions that the car puts out. Most often it fails as a result of a bad O2 sensor telling the car that the air/fuel mix is different than it is and the car dumps more fuel thatit can burn. Eventually it gets in to the cat and clogs it up which causes the honeycomb structure inside of it to deteriorate and break apart. Most likely this is where you loose rattling pieces came from. While the car is there you might as well have them pull the malfunction codes from the computer and see if any of the O2 sensor codes come up. That way they can replace them at the same time. But only if the warrenty covers it, if it doesn't let us know and we'll direct you to much cheaper alternatives than the stock parts.