You've got a bearing problem, either the input or output shaft bearing in the ends of the case, the cluster gear bearings, or the "pocket bearing" where the two shafts meet in the center of the transmission. If it makes noises in 1,2,and 3, as well as 5th., that's also a sign of input/output bearing problems. The gears are not loaded heavily in 4th, because the power goes straight through instead of taking a detour through the cluster gear. This makes 4th. a much quieter-operating gear.

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