Gearbox sometimes 'Clicks' heavily

Recently my Jetta, which has only done 58000 miles developed a clicking sound, a bit like a faulty wheel bearing, except that it was from inside gearbox. I don;t know how long it will keep going.... maybe years as it is strangely intermitent.
Anyone know what it might be, do I really need a new gearbox, and if so where is the best place to find one?

could be a diff rivet broken which is quite common or a chipped gear
get it looked at before it becomes unrepairable

Thanks for the reply. I have taken to garage and they suggest a 'new' gearbox, as reparing the old one is a very labour intensive, and possibly more expensive option.
Sometimes it runs quietly, or if noisy, can become quite if it goes over some bumps- not necessarily logical!

Diff pin sliding out and eating its way outside!