2002 qx4 4wd issues

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Hello I am new to the forum so I hope to get some advice on what's going on with my 4wd system. Does anyone know why I am hearing a high pitch whinning sound when I put my truck into 4WD LO? No check engine lights are on or 4WD lights other then what's suppose to be on. Kinda sounds like a turbo whinning up as I accelarate and slows when I come to a stop. It seems to operate fine in 2WD/auto/4WD HI. Any insight on this would be appreciated. Thanks!