The sucking sound is perfectly normal, its caused by the air movement from the money leaving the wallet area to anywhere on or in the audi. Many audi drivers have experienced this, but its not anything unusual.
The sucking sound gets louder as you begin to add mods to the car as well. Also normal.
There is no known solution.
You may be able to quiet it some by getting a new gas cap, but you will hear the sound again as you whip out your wallet to pay for it.