Transmission whine diagnosis help

Hi all, I've been off the forum a while, but the Tundra's not playing nice and I'm looking for diagnosis help, so......
I have a whining noise.
It is non-linear; pitch goes up and down, but not directly with speed.
It reacts to the throttle, it is there but quieter than the engine when accelerating, audible when cruising at highway speed, and if I let off the throttle it sort of pulses.
When I am braking, it whines down.
If I am hearing the whine, and pop the truck into neutral, the sound disappears - this is what makes me think transmission.

Any thoughts appreciated, sound didn't record well enough to upload.

Edited to add, unaffected by 4x4 engagement/disengagement. And PFA

 

Have you tried turning off all electronics (stereo) to ensure there is no interference coming through speakers?

Have you listened to engine under hood to try and detect any bearing noises from driven accessories (idlers, tensioner, PS, AC, Alt...etc)?

 

I would think alternator at first, but no whine in neutral is interesting. Is it any louder/quieter when in specific gears? Like, 1st & 2nd by themselves vs in drive?

Have you checked the fluid level & color?

 

Not affected by the stereo.

Nothing under the hood, and I replaced the alternator recently, no change to the sound.

Fluid level and color fine, I've done drain and fill on last 2 oil changes.

1st and second on their own do whine. Thanks for the suggestions!