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Sounds like a bearing issue. In particular, the bearing that goes in the rear of the input shaft/ front of the output shaft. That's the only bearing that is totally stationary in 4th gear. 4th gear is 1:1, so the input/ouput shafts turn at equal speeds.
I don't know of anything that will quiet the noise, short of replacing the bad bearing. I do know it will only get worse the longer you run it.
This bearing can suffer from lack of lubrication if the vehicle is ever towed with the rear wheels on the ground without disconnecting the driveshaft.
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