clattering from the cat's

Well the problem im having with my buddys truck is we narrowed the noise to the first cat and it makes a loud clattering from with in. Its a 97 f150 with the 4.6. We know its not the engine but we dont understand why it makes the noise in the cat. We looked and didnt see anything loose so now we're stumped. We pulled out the O2 sensor and everything was fine with that. The only thing i thought it could be is the stuff in the cat broke and is now making the noise but im not sure. If anybody has ever had this happen to them or knows what it could be please let me know. Thanks for the help.-Chris

does it go away with more throttle? "off idle"

sometimes it does but the rest of the time it gets louder. is it possible that the stuff in the cat could of broke up?

it maybe, but I would think that it would be in many pieces to make a loud noise. can you get it off and shake it?

Yea I can I just didnt want to have to make alot of cutting on it but it looks like I have no choice.

Make sure its not the heat shield that is rattling before you start cutting :-o