This happens with my car (whistling in all gears) and i used to worry about it, but when my bf was trying to sell my car, he told the most interested buyer about this, who had owned 2 subarus previously and one currently. He said 2 of his had done this but had caused no tangible problems, and said it was just a subaru quirk. Since this only happens to me when the car is being driven properly (maybe a little hard) i don't worry about it anymore, now thinking of it as a happy sound, whistling past the other cars. And hopefully i won't have to worry about it in the future since i decided against selling.