Spy Photos: Audi S6 Based Mule

What is Audi cooking with this Audi S6?

When we first saw this Audi S6, we reacted on the sound it made. It was very loud but didn’t look that different when it was coming towards us.
But then when we saw the back of it, the more central mounted exhaust pipes really made us understand why it was so loud.

As we can see, the pipes look very provisional and there is also a strange box under the car on the left side next to the exhaust.

Since we didn’t spot any other changes, the question is, what is Audi testing with this prototype?

Maybe some of the readers might have some ideas or clues.