Its probally just worn gears with age and klm , It wont affect any performance just be a bit noisy . A very broad example a W58 lasts about 500000klm driven normally and 250000klm driven hard , they are generally throw away material after that as the gears are worn out.
With some of the W,G and R boxes made after about 2000 depending on the factory/plant ( guessing) that they were made in some noisy from new with gear whine.
Dave
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