The gears are chipped or cracked.

Overhaul the transmission.

Free running gears are seizing on the thrust face.

Overhaul the transmission.

There is lack of backlash between mating gears.

Overhaul the transmission.

The floating gears on the mainshaft have seized.

Overhaul the transmission.

There is a lack of transmission oil.

Fill the transmission to the proper oil level.