Sounds like the bearings in your AC unit. It has to do with something hooked to your serpentine, and since you replaced everything else, you go with what you didn't. The fact that your AC is not working right should tell you something. Have you replaced the belts? Inspect the belts and the pulleys. It could be that wear is causing this sound, but it is more likely to be something that the belt is driving.

I would check out alternator it sounds like a bad bearing in alternator or compressor you might try isolate the noise l know it is hard to find the problem

let the engine cool down and remove the belt check each pully for excessive play if it moves more than the thickness of a dime replace either the pully or the item that it goes to example the water pump or an idle pulley also make sure you have oil and water at the proper levels as wel.

It sounds like either the A/C pump bearings, like jeligula mentioned, or one of the idler/tensioner pulley bearings. To find out exactly you can use either a stethoscope, or my favorite, a long rod. (screwdriver, ratchet extension, piece of wood) hold it on various parts of the engine, Cup your hand over the end, then press your ear to the top of your hand. Try this on the a/c pump, mounts for the tensioner and idler pulley, water pump, etc. do not put it right on any moving parts. You will hear a big difference when you find the part that is singing.

Most likely you have a bad a/c compressor. Get in your car turn the a/c on wait for the sound to start; turn the a/c off--if the sound stops its a bad compressor, if the sound stays on possibly it could be the power steering unit. Most likely from what you are saying, I'd bet it is the compressor. Another thing maybe it is a bad harmonic balancer.

Easy, the crankshaft balancer is coming apart. Pull the wheel off and the and have a look through the wheel well at it very closely.

First off, thank you for viewing.

Heres the video: https://www.youtube.com/watch?v=qHuzKRHIOxI

As a thank-you, the first one to give me a correct diagnosis, I will send a few bucks via paypal, because i'm at my wits end with this car...

Obviously this is a very loud squealing sound, and no one seems to be able to give me a correct diagnosis. So here we go.

The squealing sound seems to be coming from the belts/pulley.

It does not start right away when you crank the engine, or for about 5 minutes after that. It's only when the engine reaches operating temp for a few more minutes that the awful sound begins.

About my car:

Its a 2000 Pontiac Bonneville with 137xxx on her.

I've put a brand new belt, tensioner, idler pulley and water pump on her.

Now, the AC does not work well, or not all all.

Could this be the culprit?

Have you ever hear something this loud?