I know about a problem that occured on older 4T60 transmissions. An audible whine would issue from the transmission due to a faulty vacuum modulator valve.
This transmission is a 4T60-E or a 4T65-E, if I remember correctly. If so, it may not even have a shift modulator. Try sticking your head into the engine compartment and listening for the sound while an assistant revs the engine in Neutral. You did say you could hear it in Neutral when revving from within the car so the same should be true if you have your ear next to the tranny.
I am however inclined to think that this is actually NOT a tranny issue. You said you hear it with an increase in throttle. It may very well be in the intake setup. A leak at the air box, or even at vacuum, could make a whistling/whining noise. I’ve heard of old dirty air filters doing it too. Also, does it whine with the radio off? It could possibly be electrical; been there and done that.
-Matt