2000 s10 whining noise

2000 s10 4x4. Makes a whining noise while driving. Just replaced front bearing assy on both sides. No difference. While driving the other day I put into 4 wheel with no luck. The whine isn't always there. While driving if you put the switch into 4 wheel the whine goes away, however its not in 4 wheel drive. Transfer case switch? I put one in a few years back....any ideas? The whine sounds like bearings but makes no difference when turning either way