Originally Posted by SweetZRag

Another possible cause of the rattle is the element inside the catalytic converter. It sounds like bolts in a can. Try having someone step on the gas while you get your ear near it.

The element can break loose from overheating and the overheating can be caused by the engine running too rich causing too much unburned fuel to reach the element. If this is your cause, check to make sure the engine is not running rich or you will just replace the cat again in a few months.