Making a whining noise when accelerating?

Lately my vehicle had been making a whining noise when accelerating. It's noticeable right before I shift from 3rd to 4th, and it continues until I slow down or stop. It also stops when I press the clutch or it's neutral. If I'm parked and I rev up the engine I do not hear it. It doesn't sound like it's coming from the engine but somewhere closer to the middle by the transmission and transfer case.