I have a 96 A6 2.8 196k miles
There is this noise coming from (i think) inside one of the catalytic converters. It is a rattelingsound much like a lose flange would make on a exhust pipe, but nothing on the outside looks loose. Has anyone heard or fixed this kind of noise. Also there are what looks like plugs on the back side of each cat. what are they for?
thanks

 

Take off the exhaust band clamps under the car and take it for a spin around the block. The noise should be gone.......well except for the car being extremely loud that is.
If its gone, replace the band clamps.