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I hate to suggest it, but its possible that one of the needle roller bearings in the back end of the gearbox is failing. A Gear in the shaft will then drop slightly and be hard to get in gear if not impossible, and will be noisy. the tractor has to be split next to the back axle, hydraulic top and gearbox top will have to be removed first to take selector rod front end off. Allow a good day to do it if cab bolts are not corroded as mudguard extensions have to be removed to roll backend with its wheels on.