Just an update which is somehow related to your reply. The car was serviced today and they told me that the rear pads were stuck in the career and it was the reason for the noise. They removed them, cleaned them and greased them. The noise was not there for about 50miles but it came back after. It maybe better now but the noise is there. The fact that the noise disappeared because of the greasing may prove the problem is from the brake pads. I still do not know why then I even heard it at high speeds.
So, maybe the best solution is to change the pads with non-genuine ones. Any ideas?