Could possibly be pedal if self.
The cylinder piston joins onto pedal through bulkhead. Where it joins pedal is a box housing. It's sometimes common for the metal box housing welds to break.
Have a good look at the pedal.
You can buy a new pedal and white clip. Roughly £25 from Audi or TPS.
Or you can remove the pedal and fix the box housing by straightening and rewelding (extra welds to make it stronger). Then all you need is to buy a new white clip.
Removal is fairly straightforward but refitting pedal is a pain due to the tension spring and getting new white clip back onto ball joint on end of piston (tip... Place new white clip into hot water for awhile to soften but placing back onto piston end).