Exhaust noise like an E46 M3

[BaransuIS250]

I was driving the S3 behind my wife in the 200T yesterday and the Audi isn't particularly quiet. When she accelerated I could hear quite a loud hissing sound! We then went out again later that evening and listened for the noise. After some swapping of her driving and me listening and vice versa it seems to be coming from the rear exhaust. The front is silent when accelerating but the back hisses, reminiscent of the old BMW M3 E46. I don't remember this happening before - is there a cure? Any 2013 on- IS250 owners who have noticed this too?