If the substrate inside the cat(s) goes bad - they can break-up or get loose. It's not unusual for the padding around the brick to fail & then the brick gets loose inside - sometimes they can even turn sideways & block exhaust flow. I had a '96 Ram with a bad cat & it rattled like heck on a cold start, but then when things heated up, the rattling stopped - it never threw a code though. Dodge replaced it under warranty - the OE cats are covered for 8 years or 100,000 miles federal law. When the truck is cold, get under it & put a glove on & bang on the cats & see if one of them rattles inside. If it does, the brick is loose & it needs replacing.