The MT 75 box is still used in the Transit and the front layshaft bearing falls apart for fun in those , try driving in 4 th gear and see if it is quieter , and then in third and see if it is at its worst , as in 4th it is at its least loaded state and in 3rd its most loaded .

Also agree with marco as if it was oil it would be notchy in all gears not just second , would suspect a selector or a synchro hub / baulk ring issue to be the cause of this .

Hope this helps