Car

Is the noise concurrent with wheel speed?
Or can you hear it change as the engine gets higher in RPMS?
My first thoughts are to check fluid level, especially p[ower steering and the transmission.
Also, check the coolant level.
Has there been any other issues with the truck in the near past?
Let me know and I will respond ASAP.

Could be a vacumm leak or the radiator cap could be leaking.
Does the noise start as soon as you start the vehicle?
If not, the radiator cap might be whistling once the engine heasts up and the pressure is leaking out form its' seal. In that case replace the cap, they are pretty cheap. Otherwise, with the engine running open the hood and listen for a sucking or whistling noise. If you can get the general area from where it is coming from, get a string and start dragging it near any parts connections, i.E. Throttle body to intake, PCV seal, etc. And it will get sucked into the the area or conncetion that is leaking.
Let me know if that helps.

I guess I did not convey what I meant by concurrent with the vehicle or engine speed. Sorry, communicating via text is tough.
So, the noise only occurrs when the vehicle is in motion?
Does the noise change as the vehicle increases in speed?
It could be the axles, but it sounds like the transmission.
The trnsmission fluid, is the level ok? Does it look or smell burnt or diry?
Look at the tranny fluid in the light and look for tiny metal particles, see any?
I will look for your reply.

Since the fluid looks good I would eliminate other possibilites before spending the money on the transmission. If the noise sounds like it is coming from the wheel area, the axles or wheel bearing probably need replacement. If the axles have never been replaced, that is more than likely the problem. Bearings tend to las a long time unless they see svere use.
Front axles are not too hard to replace. There might be a tutorial on this site to walk you through it. Although it will be generic and sometimes all wheel drive vehicles can take a little more time as there are more compnents involved.
Go online and get either re-maned or new axles and look for a good brand. A1 Cardone and Beck Arnley are 2 reputable aftermarket remanufacturers. There are links on this site to help you find parts shops that are reputable near you if you want to buy them at a store. This can be advantasgeous in case something happens like a defective or inccorect part being sent. Also replace the axle seals and tranny fluid at the same time. When you change the tranny fluid, use a clean pan and when you have drainned the fluid into a cojntainer, leave just a bit of the fluid in the pan and inspect it for small metallic particles. Don't panic, failure is not imminent, some meatl is normal but al lot or large particles should be soething to look at and addrress if need be.
To replace the axles you will need a lrage socket and impact wrench or breaker bar. If you don't have these, take it to a repair shop as not having the right tools and a manual for specifications like torque and capacities, can create a big problem. The site can guide you to a reputable repair shop as well.
Hope this helps.
Dr. C

That is strange, but odd thinhs can cause odd noises.
Do you think it has anything to do with the thermostat? Meaning, fluid being foorced by it and creating a whistling noise?
Or maybe they leaks slightly and the whistling was steam eascaping, like a teapot?