transit crunches into third gear?

my transit crunches into third gear its a 2001 fwd duratech 2ltr 5 speed. All other gears or great and once in third goes great. No other noises from the gearbox. Just the crunch/grate as it pops in!!! has been mentioned to check the cables first, may have stretched or broke, were are these ans what do i check... Any help appriciated