Huh? What are you hearing? Unless it is a CD that you can feed in line levels, I can't imagine a way to do it without a mic. How noisy is your studio environment that the background noise is that bad, and again, what are you "hearing" that background noise would really have any sort of effect. I mean, of course, if you are recording spoken word and live near a busy railroad yard.....

Just a little nugget for you.... I found 2 doors on the free section of Craigslist. I laid those doors flat on the floor and took a thick shipping mat and stapled it to the doors. When I record vocals, I take that gabo and place it against a corner of my room so the 2 corners of the room and the 2 faces of the door make a 3x3 ft booth that is almost completely anechoic and very quiet.

Until the train goes by.....