Had a fiesta zetec about 3 years ago like this, drove us nuts.
Turned out to be VERY small pieces of old belt lodged in the crank pulley.
Cleaned the grooves, all OK.(just a thought)
Coming back to this A4 though, was the damper for the tensioner amongst the kit from LUK. Because these are the parts Audi have modified. Part no.s: Damper............. 038145299A £79.62 Modified bracket... 038145283A £37.72
If you have not replaced damper, pull it off and inspect closely, the bolt holes DO elongate.