How can i sort out the Rattling noise from the gearbox on micra?

Hi, my 1998 1.3slx auto micra seems to be making a rattling noise, seems like a loose screw, showed it to a guy who said its the n-cvt gear box, how can i sort this problem out and how much would it cost me, the gear somtimes feels a little bit difficult to change but only now and then.. thanks