High pitched whining noise

OK so as the title says I have this odd whining noise that sounds like it's coming from the front end.

Truck details. 04 tacoma 4x4. 2.7L, 5speed.

I just started noticing this about a month ago. In the mornings when it's cold I'll start to drive and when I reach about 40mph I begin to here this weird whining noise that changes quickly/rapidly in pitch. The noise almost remindes you of an electrical interference. You can only here it when the radio is off and hvac motor is off. As you increase speed it gets a tad louder and higher in pitch. Slow down it gets lower in pitch and disappear below 40 mph. If you keep driving for about a mile it will disappear all together and will not come back until everything cools down again.

I've tried driving straight ahead and disengaging the clutch to see if it was engine related and it is not. Changing gears will not make a difference. Only when I engage 4wd will it disappear. Disengage 4wd it will reappear. Or if I keep driving and let it warm up will it disappear. Any ideas on what it is?

I've recently replaced pads and rotors and brake lines (it was already making the noise prior the brake job). Just had an inspection and it passed. Drivers side wheel bearing was replaced a couple thousand miles ago. I had a master rebuild done on the front and rear differentials (changed gear ratios) a couple thousand miles ago. And the drivers side axel shaft was replaced a couple thousand miles ago due to a boot busting. So the only thing that is original is the passenger side axle shaft and passenger side wheel bearing. Any thought?

Thanks in advance?