Thank You for the answer but with all due respect, I don't think that is the problem, because i tried with other outlet and the buzzing remains.
But today I put some duct tape on 5V pin on usb cable, and the buzzing sound is considerably lower, almost non existing :)
It appears now only when i turn the pot all the way, which i never do when listening.
So my next step would be to cut that wire which is responsible for 5V in the cable, and then I hope everything would be allright :)
+5V with a ground loop relates directly to it.
It isn't another outlet you need to try, you have to use another circuit (aka try a different room). If that can't be done, Heatfan's method would work.