I was coming home from the TA Nationals today when a shredded truck tire was kicked out in front of me by a another truck. I couldn't avoid it and I ended up running over it with the passenger front wheel. Fortunately there was no body damage or suspension, but my one cat may have taken the brunt of it. The car drove normal until I pulled off the highway and slowed down. I noticed a rattling sound coming from under the car, like a can of pebbles. Also, I thought I had a big exhaust leak as the car was very loud under acceleration and had a major hissing sound. Anyhow, this happened close to home, so I got it in my driveway. I let the car cool down and then I went to take a look. I could see no signs of damage, so I started the car up and it seemed normal, no leaks but then it started rattling pretty bad. Could I have broken something loose inside the cat that has somehow plugged it? Why would it sound like an exhaust leak? Could there be something else wrong?

The inside of the cat can come loose and rattle around...My truck did this, not from an impact, but just from age. Truck drove fine, but when I was idling in gear with my foot on the brake it would rattle like crazy. I hit the cat with a rubber mallet and was able to replicate the rattle, so I had it replaced.
Could be the muffler as well, maybe one of the baffles inside got knocked loose....

I just wonder why it has an exhaust leak sound and yet I don't see any holes. The rattling is definitely coming from under the floor pan area by our feet. I will try the rubber mallet though.

If the air can't get through the cat back pressure will rise and it will leak where ever it can get out. Get the cat fixed before you cause more damage.