79 series cruiser gbox noise

i have a small clunk type noise comming from my landcruiser gearbox. it only happens when i put the gearstick in first and then releas the clutch. and it only happens when the oil is hot. its not very consistent though, sometimes its quiet and sometimes its louder and makes the gearstick slightly bump to the left a bit. ( about 10mm measured at the top). has anyone had this type of problem. i am 100% sure its the gearbox and not the diff , transfer case or drive shaft.!