1999 Chevy Suburban whinning nosie while driving

When I driving down the road there is a whining nosie coming from the transfer case the front driveshaft isn't spining in 2hi but it makes the same whining noise like the truck is in 4wd. The 4wd does work well still. There isn't any banging or grinding when going into 4hi, 4lo, 2hi any ideas