Strange noise when downshifting

[PGabriel]

I´ve had my new E90 DCT for about two weeks and today when downshifting from 3rd to 2nd in S4, as the RPM went up to about 4,000 there was a noise like a single crack/blast coming from beneath the car.

Could this be due to fuel bad quality? I always fill her up with one of the best kind of gasoline existing here, as recommended by the owner´s manual.

Note: still in break-in period and driving according to the book.