there can be a number of causes for your noise that would require dismantling the rear of the tractor. but before you do that, change the fluid in the transmission and final drives using #90weight and see if that quiets it enough. you can go a little deeper by removing the shifter cover it's a simple way to look at the transmission. just remove the 10 or so screws in the cover and lift off. that will tell you whats up... ie rust, water, gunk, look especially at the oil gallery for the top shaft rear bearing.
if you have water in the transmission, a fill with kerosene or diesel and run it around will clean it up. then refill with #90 gear lube.
hope this helps,
otherwise, time to get greasy