Newbie here...00 Marquis Cat. Converter issue
 

Just traded my truck for a 2000 Marquis...Its a very nice ride but the check engine light is on and it says its the cat.. The passenger side cat. makes a rattling noise but it almost sounds like the o2 sensor might be loose. Could a loose o2 sensor cause the catalytic converter code or is it definitely the cat? Any help is greatly appreciated.