I have exactly the same problem. They kept it for three days at the mechanic's workshop, drove a lot of Km and when they gave it back to me they said the problem was due to part of the roof covering the predisposition for side bars and that they had fixed it. However, the problem was unchanged! When you drive faster than 90 Km/hour it is like being in the middle of a wind storm
I will go back to the mechanic on Monday but I have little hope of getting it fixed.
If it stays like that I will have to sell my car
For the rest it is a good car by now (I have driven it for 4000 Km)