Rattle noise on 2nd gear
 

I just came back from the track and I noticed that when I'm not pressing on my gas on 2nd gear, it makes this rattle noise when I'm going slow. It's louder when it's cold and as it warms up, the noise gets quieter, but it's still there. It's only on 2nd gear and not any other one.

Anyone know what the problem could be? I'm hoping that I don't need a new tranny!