My mates old Kinga HZ wagon had the same problem, what had gone wrong was the Thrust bearing in the gearbox was F*&ked. Doesn't cost much to replace, I would fix it before the bearing goes to pieces and stuffs the g/box (busted bearing pieces in a g/box aint a good thing). I was told its from loaing it up on take offs too much, might be wise to try and get someone to beef it up a bit?
Cheers
Sumo