Strange Noise from Trans OR?

1997 - 5.7L - 4L60E - half ton - 4x4
I have noticed a strange noise twice now. Both times after the truck was parked outside in the cold (0 to 30f). Sounds like a low whistle or whine. kinda like when you cup your hands and blow thru them to make a dove call.
It comes from the rear of the engine or front of the trans. Mostly at lower speeds (20-40mph). It gets higher in pitch as the rpms go up and goes away over 2000rpm or so or 40mph+. It was very loud and whine-y the first time, sounded like the trans, but i only moved the truck, and did not drive it. The second time, sounded more like a whistle coming from the intake while driveing as described above.
I checked the trans fluid. It looks ok. A little overfilled cuz i added a quart of trans conditioner a couple yrs ago. I also checked the air boxes for cracks, none found. It is not a belt, or pully, I have had those squeeks too and this aint it. Oh, doesn't sound like a diff noise either.
I dont drive the truck much anymore, use it mostly to tow my trailer so I haven't had a chance to do more investigating. Wanted to get some ideas about what to look for. Any thoughts?

Alternator will whine when it's on it's way out. But being it seems you pin pointed it towards the trans, I'd say the Torque Converter.

check the U joints. they will make many sounds when begining to fail. cold weather/not using often can cause these to fail . if parked over unpaved surfaces mositure gets into them.

see if the joints are discolored. operate vehicle and see it one is warm or hot compared to the others.

with the transmission fluid. when adding additives to aid in the transmissions performance it is best to replace the fluid shortly after. these products help remove deposits in the torque converter and the many valves/passageways. use the new dexron VI. do not use dexron III.

If you over fill use a hand held pump that is for pumping from an oil container. use a hose about 3/8 dia 4 ft long and stick into the fill/dip stick tube . I have a clear hose on my pump and this pump also fills the diff/transfercase / and my mercrusier outdrive..

on your transmission at the fill mark you can suck out about 1qt...

i have seen a pcv valve ice up from moisture in the system, can easily be checked my plugging the line as a test but they are cheap so you might just want to change it. it can make a noise like that.