This is a 03 with 3.9 rear gear and Mismo LSD. It has 6000 miles on the rear pumpkin. I purchased it from 350evo as a built unit. I ran normal gear oil until just changing to Royal Purple and a LSD tratment. The noise developed some two months later and seems to be getting louder. It only is there from 40 to 60 MPH and is not shift related. It also stops when I drift. This seems like gear load issue.

Has anyone had this problem or simmilar experience? Please share ideas.

 

What does that mean?
Try Motul from Sharif@Forged. The testimony can be found here, including mine.
https://my350z.com/forum/engine-and-drivetrain/235891-motul-diff-oil-with-nismo-diff-review-inside.html

 

If it were mine I'd drop it out and check the backlash on the ring and pinion.