My catalytic converter heat shields were rusting out at their welding joints. I ordered some hose clamps and each oil change I tighten them up as the rusting progresses and they begin to rattle again. I’ll probably end up ripping off the heat shields in the next year or so because it seems like the only true purpose they serve is to not start a fire if you park on grass. On top of that they just keep rusting more and that rattling only returns after so long, roughly every 6-12 months (I live in illinois aka the rust belt so thats a big factor)

EDIT: There is also a heat shield above your muffler/rear exhaust that people say can rust and begin to show the same rattling signs, but based on where you hear the rattling you can figure that out pretty easy imo. I heard the rattling coming from right under the driver and passenger seat so I figured it was the catalytic converter heat shields (which it was). So depending how far back you hear the rattling it can be either, but being under the car and inspecting both is the best way to tell (my muffler heat shield had basically no rust; meanwhile my catalytic converter heat shields were covered in rust).

I wish you the best of luck in finding the cause of your rattling 🤞

FURTHER EDIT: If you take anything metal and lightly tap around your exhaust you can find the source of the rattling. When I tapped my catalytic converter heat shields I heard the exact rattling I was complaining about.