WE had just bought it and drove it 150mi. when noise started. Sounded so bad I was afraid to drive it to machanic. So if you hear what sounds like a knife thrown into a disposal, it may not be so bad!!
It was just the catalictic converter. After market one and all wrenching got it fixed for $300. Yea.

 

Good deal. I would point out that listening via a long stick works for things under the truck as well.

 

Sometimes they will rattle on de-acceleration only. Mine did this the day I got it home and it sure sounds expensive.