1980 240D non turbo 4 cylinder manual diesel.

I can't figure out what is going on and how to check it. Whenever I shift into 4th gear it makes a quick short grinding noise. It used to make the same noise when shifting into 3rd gear, but somehow it suddenly stopped after I fixed the vacuum leaks in the car. There was a leak in the door lock system so I bypassed it. Now the grinding happens only when shifting into 4th gear.

Can someone please explain what is going on here?