I was driving my car today, and i was driving it a little hard. All of a sudden my exhaust tone changes to almost like a low pitched hallow tone. Then when i was on my way back to my house because i didnt want to drive it while it was sounding like that it sounded like my exhaust was rattling really bad, so i figure something had to be loose. When i got to my house i got under the car checked to see if my exhaust was loose anywhere and it was tight everywhere i checked. When i started up the car to listen to it while i was underneath it, it sounded like it was coming from the driver side cat. Is it possible that the internals to the cat have broken and thats what changed the exhaust sound and whats causing the rattle?

Any thoughts would be greatly appreciated.