Oh it just started?
I got my car second hand and it always sounded like that and had it checked out and it turned out to be fine....
I wonder...and this is just a wild ass guess...if BMW slowly over a period of X number of miles increases the fuel pressure on these injectors? So that initially the car starts off quiet and that over a short period of time the pressure increases to it's full capacity and then the sound starts?
A friend of mine works for Volvo and for their new 2L T models that are coming out sound was a major issue with respect to their DI system. That tap tap tap was freaking out the Alpha testers.