Rattling noise on my 1999 Honda CRV?

For the past couple of months my car has been making horrible rattling sound. It doesn't happen just when I drive, but as soon as I turn my car on, when I'm in park, and then always when I'm driving. It's constant. I have a 1999 Honda CRV automatic with 210,000 miles (I know, a lot!). The best way I can describe the sound is that it sounds like a part is hanging underneath and is rattling all over the place. I was wondering what you think the problem is or how to go about fixing it… And how much it ll cost me! It doesn't seem like it's harming my car, just my ears.

It's been going on for months what is your hurry? Maybe in the spring you can get under and see if something is loose.

Maybe catalytic converter, if indeed catalytic converter then that's expensive, maybe above 800 usd plus labor cost. Catalytic converter reduce pollution, not damaging to driving system

Probably just the heat sheild. Thin piece of metal under the car to separate exhaust pipe and floor of the car. They rust easily and come loose all the time on older cars. Not really a big deal.