Any noise from the back, especially a shifting noise when accelerating/decel... points to fuel pumping.. I just had this.. ONCE YOU START HEARING THE NOISE, it is time to change the filter. The noise is comming from a clogged filter causing the pump to work harder.. Also, to my understanding, there are 2 filters, on external and one internal to the tank. The external is on the outside right next to the tank, in the middle of the passanger side.
It is expensive if you go to a dealership. The one here in St Louis wanted $1100, the Midas wanted $860.. I had my Audi guy (30+ year retiree from Audi Service Dept) get a fuel pump for 230(oem) and filter for $45 and install them for $100.... It is work at a medium difficulty. There is a place in the trunk that gives you easier access to the fuel pump, which is in the fuel tank. You have to remove the spare cover in the trunk in on the upper-right of the spare compartment, there is a removable plate that once removed, will give you access to the fuel pump. I watched on how to do this so I can do it next time....

 

Thanks for the information JonCowden,

But hopefully there won't be a next time!

Audi dealer is planning to check mine out this week.