Its hard to describe the fault with my gearbox, i had my dad under the car and I was replicating it whilst up on jacks, he confirmed its definitely coming out of the transmission. I don't think it is simply a case of it being noisy or that its a 350z 'feature'. What I have is a physical movement within the box, an actual clank of metal vs metal, like there is a gap between the gearing and its moving about inside the enclosure. Its most prominent when I drive in 1st gear along the road, if I let off the throttle it makes multiple clanking noises as if two bits of metal are just banging against each other.
I'm not sure fluid will solve this issue, it certainly is a mechanical thing I would have thought?