Start with the simple things, change the oil
and inspect it closely for metal particles, check the position and travel of the selector arm. Get a flashlight and take a peek through the cooling
air grille and inspect the drive plate. Some of the engines that had this gearbox
also had a cooling
fan attached to the flywheel, it could make a whining sound..... itís a big fan although if thereís nothing in neutral or reverse it does narrow the field a bit.
Hereís one that had me beat for a while.....same gearbox
, noisy in forward but quiet in reverse so I undid the coupling to eliminate the propshaft as the culprit and the noise ceased BUT the gearbox flange nut and shaft was unusually shiny. Turns out that the shaft had moved forward in the coupling or had been assembled that way in the past and the propshaft was hitting/butting the end of the gearbox output shaft. We moved the coupling 12mm further up the shaft, re joined the coupling and the noise was gone and to this day I canít explain why.
Logically that shaft and the flanges should work
as a rigid assembly and it shouldnít matter that the shaft butted the gearbox but it definitely did.