Suealing in the motor

if not alternator ,power steer pump,air con ,water pump \
serp belt tensioner or idle pullys

check crank pully is not running out of true

could be serp belt squeal
replace the serp belt

Year make and model ??
The belt could be glazed and slipping even if it is tight.
You could have a weak belt tensioner too.
The noise when the ac or heat is turned on can be the extra load from the ac compressor or the alternator. The compressor cycles on and off especially in defrost mode.
The load from the alternator is the first noise of the day. As the belt warms up it becomes better at handling the load of the accessories.
Even tho the belt looks fine, you may have to replace it to get rid of the noise.

Have your water pump and power steering pump been eliminated as the possible cause of the noise?

There are only 4 things that will cause a belt to squeal.
1. A bad/worn-out belt
2. A bad or misaligned pully or pulley bearing
3. A bad belt tensioner. (pulley, arm, pivot, or spring)
4. Malfunctioning or bad component (alternator, power steering pump, A/C compressor, etc.)

Remove the belt and check ALL pullies to make sure that they rotate smoothly and do not have any horizontal play in them (indicating worn-out bearings). Make sure that the tensioner pully swings freely and does not bind in its travel at any point and that it is properly aligned with the other pullies. A good way to check for proper alignment is to look at the belt position on the pullies with the engine running. If the belt is running on the edge of the pullies instead of in the center, it is usually caused by a bad belt tensioner or a bad bearing on one of the other pullies.

Check the belt for chunks missing, torn or cracked ribs or shiny "glazed" edges, ribs, or back.

You state you changed plugs and wires - Do distributor cap & rotor? If not you should then set timing. That should cure hesitation (might change fuel filter also and run good fuel system cleaner through it)
Prob 2 - sounds like a u-joint or possible brake pad/rotor issue. Does the squeal only happen in mornings when cold? Sometimes after sitting overnight, rotors get condensation and when backing out they'll squeal/grind. Somethings to look at...

The squeal is coming from the alternator belt, When the temp is low, the belt is hard and glazed and lost its ability to create friction.

Take a spray bottle of slightly soapy water and spray the pully.If the noise goes away or gets quieter you have your culprit. If not work the other bearings one at a time with the same method untill you get it.hope this is helpfull;good luck

It could be a pulley. Best way to tell is to remove the belt and turn each pulley one at a time and see if you can either feel one not turning freely or try and duplicate the squeal. Keep in mind there is no load on the pulley without the belt on it so it might not be as pronouce as it would be with the belt on.

To loosen and remove the drive belt on a 2004 Toyota 1.4L engine you simply use a wrench and pull up on the spring-loaded tensioner.
You should be aware, however, that if you are experiencing problems with belt squeal or rattling noise, there is a Technical Service Bulletin (TSB) for your vehicle that provides information for an updated tensioner assembly.

well also it could be your a/c compressor