I can't think of anything in the gearbox which would only tick in 3rd, 4th and fith as all gears are in use except in 4th which is normally a straight through drive, the only thing that I would think could cause such a noise would be a broken tooth in which case the cheapest way is to fit a secondhand gearbox.

Good evening

my lovely old Toyota has started to make a ticking noise when driving straight and accelerating hard, in 3/4/5th gears. The sound comes from around the passenger well, its not the cv's, possibly gearbox. I use the van for towing a caravan. Any suggestions or possible cures please. Shes old and has done 122K.

Peter.N.thanks for your advice