My catalytic converter rattled in another car of mine. This first thing I have to ask you is, has your truck had any emmision problems in the past? for example a failure of an 02 sensor or mass air flow sensor? Or does your truck sometimes smell or in the past smell like rotton eggs or a sulfur type smell? My car had these emmision problems which made the catalytic converter heat up to very high temp and caused the honeycomb type filter in there to give away and break apart, hense the rattling sound. I was told after an inspection that it needed to be replaced and it was very expensive. The job at the dealer was nearly $1600 parts and labour....I was lucky the warranty covered the problem up to 190,000 kms
(VW's good service).
You have 2 options go for the dealer part for a longer lasting catalytic with a new parts warranty or go for a cheap aftermarket one which may last you 5 years. Ask yourself how long do you plan o keeping your vehicle..?
Hope this helps..Sorry for all the writing...