The catalyst is a very porous, soft rock and they do break loose and can rattle. Bang on it with a rubber hammer and you'll hear it rattle if it's loose. The proper repair is to replace the converter, but before you do that you can try a temporary fix. Drill a small hole in the bottom of the housing, start a self-tapping bolt to make threads, then take the bolt out and grind the point flat and smooth. Run that in just until it won't rattle any more.
The catalyst is breaking apart and chunks are going out the exhaust, and eventually the Engine Computer is going to detect that no change is taking place in the converter, and it will set a diagnostic fault code and turn on the Check Engine light. Pressing on that bolt might make it break up faster, and holding it from rattling might make it break up slower.
Thursday, February 27th, 2014 AT 8:28 PM