Hi
I'm from Denmark and I will write in English, as my German is for reading and talking - not writing 
I have read through this thread a few times after purchasing my Astra K in January. I have wind noise too. It comes from the back and the more windy, the more noise.
After visiting the dealer twice they seem to now have akknowledged the problem the second time around. Together we have pinpointed the noise to come from the space between the spoiler and the roof. After taping it over as a test the noise was completely gone. Now, however, they have not made permanent a solution yet, as they are currently asking for advice from technisians from the headquarter of Opel. Apparently so far they have no record of similar reported problems (which seems strange to me). My dealer have not (so far) been able to find any visible faults on the car in that area.
Nevertheless, I am now waiting to hear about the next move to permantly get rid of my wind noise.