Loud Rattle

3.3 liter. This morning as I was driving, the car shut off & there was some smoke that came in through the heat vents & a burning smell. I pulled over & noticed smoke from under the hood. Upon opening the hood, looking down directly in the center of the front of the bumper, there was a round 3-4 in wide x 5-6 in long cylinder that looked like it was connected to the exhaust. The car will start but I didn't try to run it because when I started it there was a loud rattle like something broke. Do you have any idea what this is or how must it will cost? Thank you.