its your input shaft bearing. they are a common problem with skylines and i dont think its that big of a deal for now. i wouldnt bother replacing it coz its gonna cost a shit load to take the box out and apart. as long as you dont go giving the box a really hard time everyday it will be right. just leave it until it gets worse then get it rebuilt or replace the box. i had that problem and the box lasted fine for a long time and it seems there are lots of others out there who have this problem and drive around for a long long time before doing anything (if they do at all).
hope this helps.
-actually the only thing i would do for now is make sure u have enough oil/replace the oil in the box.