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Ken,
Like Jim, I'd be guessing it to be a gear meshing thing. As Jim points out, some trannys and diffs can whine for years, with no serious consequences.
I'd be doubtful it was bearings as they rumble rather than whine, and get noisier very quickly. Also, if it was a bearing, the noise would follow you through the gearbox.
If it worries you, check your 3rd gear for wear through the case hardening. RR
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