There were lots of reports of this some years ago, haven't heard about it in a while so either the threads have been found in a search and answered people's questions or enough Navs have been fixed that nobody has been having the drama - until now!
It usually happens in the manual because of the hydraulic line for the clutch running down the firewall rattling against the firewall itself. Easy way to tell - grab a small amount of packing foam and slip it under the hydraulic lines. I can't recall the long-term fix, I think Nissan may have done something to change it.
If yours had been fixed and is now doing it again, there's a chance that the engine/gearbox mounts are no longer in the same position (that's a worry!). Inspect the underside, engine mounts on both sides and the cross member as well (had one member with a bent cross member but I think his was due to a rather heavy landing on a 4WD track). You might also notice the front drive shaft is showing a bit of exposed shiny near the unis (and none on the rear drive shaft).
