Could be the gear mesh but could also be a bearing. Check all the bearings to see if they are smooth. To do this properly you should take them out and check them by turning them in your fingers. They should be smooth and free.
Sometimes they feel alright if you check them in the car but under accelleration they will bind up and this makes a grinding noise.
Is your car slow..Xray it!.......FK05, M18, M18t