High Pitch noise

I purchased a 2010 Tacoma 4 cylinder, 5 speed, 4x4. Truck has 26k miles. I recently noticed a whine sound (almost sounds like a turbo) when accelerating (rpm 2500+ The higher the rpm, the louder the sound). If I put the tranny in neutral, and rev the engine, the noise does not exists. If I am driving and I push in the clutch, the sound goes way. It only happens when the truck is in gear and accelerating. So it seems like the sound may be coming from the transmission. Has anyone else experienced anything like this? Is this a normal sound for these trucks? This is my first Tacoma, so not sure what to expect.