The gearbox rumble from a TD42 is well known and attributed to "harmonic resonance", even more pronounce when using the Nissan Genuine HD clutch kit (As Pete said) , which is 5 spring ? i think. I have posted a fair bit of info on this forum about this phenomenon. But in Short,.. its only audible when the vehicle is under little-to-No load at low RPM, (as Cedric said). And it dissipates once rev's and/ or Load increase (ie: 1300 RPM onwards as an example). Its also mostly audible only in 1st and 2nd gears.
However,.. this Harmonic resonance is not something that is audible, nor related, to gearbox/clutch noise at standstill,ie: when clutch is either depressed, or released, ...which is the case here. So i dont believe this to be the same issue at hand.
If the vehicle is at Standstill, and there is audible noise / vibration when clutch is operated, then it must be something else...
Thrust bearing / Release bearing ? I'm no expert on clutch's, just contributing what i have experienced.
'07 Patrol Pickup TD42 N/A,285's KM2,3" Lift,ONCA Bars,Warn High Mount,Alu Canopy, and some other Stuff.