Catalytic Converter Rattle

I drive a 2004 ford explorer 4.0 SOHC 4wd. about a week ago I noticed a loud rattle on start up and I was convinced it was timing chain tensioner or a bad starter bendex. I finally realized it was the passenger side catalytic converter making the noise, I saw that a lot of times its the heat shield that rattles but on mine it appears the clamp is good and the heat shield is sturdy. Any idea on what could be wrong? does it need to be replaced? Thanks in advance

I've heard it could be the cat or the resonator. Posting to see any other replies as I have the same (probably) on my 99 sport.

Before you go spending a pile of money on a cat, I strongly suggest you take an hour or two and pull the valve covers. I would be willing to bet you have timing guide bits sitting in the bottom of your oil pan. Its not a long job and it would probably save you a pile of money you don't need to spend.

Passenger side cat area noise could definitely be coming from the right rear timing hardware.