2008 Ford F-250

Slight rattle when idling that sounds like it's coming from the catalytic converter. Rattle speeds up a little when rpms are raised, but the noise disappears when driving faster. Was pretty loud when I started it this morning but quickly got quieter. Noise is definitely on passenger side. Sounds like maybe something is loose on the inside of converter, but it is NOT the sheet metal covering. I have a good amount of wrench turning experience, and at first sound, I thought of a knocking rod, however the sound is not in the engine compartment, it is localized in the catalytic converter.