Solid rattling sound from lower catalytic converter

1999 Taurus. There is a solid rattling sound coming from the lower catalytic converter. I've checked the heat shield above the cat and it's solid, so the sound isn't coming from there. I know there are ceramic pieces inside and they can come loose, but this sounds more like a bolt or nut. The sound is very solid. From what I have seen of the ceramic honeycombs inside a cat, I would suspect any pieces rattling around would break down.

Does anyone have experience with this? I'm wondering if an O2 sensor has broken and that piece is rattling around. I'm going to pull any upstream O2 sensors to check them. In the mean time, I wanted to see if anyone had ideas on a solid rattling sound coming from the lower cat.

If you hit the bottom of the cat and hear it, mine had gotten gravel trapped in the gap between the shield and cat itself.

Thanks for the reply, aleekat. I finally got some time to look at this. I pulled the exhaust pipe just below the lower cat and peeked inside. It is definitely the innards of the cat that are loose and bouncing around. It looks like I'm up for a replacement.

Sell your used cat to a recyclers. Offset the cost of a new one.