I got rid off that annoying sound.
I tested it today, and I found out that it doesn't matter which laptop was connected. Yesterday I was probably so tired, that most of the sound was coming from my head..
I moved laptops place from under the passengers seat to different place and interference wasn't that loud anymore. Then I went to car-audio store, and bought ground-loop isolator.
That was the cure. Interference is finally gone, and I'm happy.
That was your first quess too, so thank you for that!!
