Loudness of co2 airsoft pistols?

i'm getting into airsoft and i want a co2 pistol; however, i have a small backyard, so i don't want to attract attention. I was thinking about getting the crosman airmag c11, because it would help me not get unwanted attention from neighbors if my gun is clear. I have never handled anything co2 before. Can you please tell me how loud they are? like on a scale from one to ten. or a comparison with another noise.