2002 Silverado 4x4 grinding noise

I have a 2002 4x4 Silverado 1500 (4.8L) and when I drive at the lower end of the 4th gear (overdrive) around 37 to 40 mph I hear a grinding noise that
seems to come from my rear axle. I goes away as soon as I slow down, go faster or shift on the third gear. Did anybody experienced that kind of problem and what was it.
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