Possible a/c problem, squealing noise constant, but gets better when a/c is turned on.

A fan belt noise would change with engine speed, not vehicle speed. A squealing noise that changes with vehicle speed would be caused by brakes or wheel bearing. Since it stops when braking I would suspect a brake issue, like a stuck caliper

Common Car Noises and Their Likely Causes Noise More Details Likely Cause Fixing It Clicking noise while turning Starts faint, gradually becomes more noticeable until very loud Failed outer CV joint (constant velocity joint) due to torn boot (most likely) or old age Replace either the whole axle assembly (recommended) or the outer CV joint Water pump noises Difficult to diagnose because of water pump's location Noise usually caused by damage to the bearings in failed water pump Only cure is replacing it. If possible do it at the same time as a timing belt replacement to cut down on labor costs Squeak or squeal from an engine belt Loud, high-pitched squeak or squeal Belts either need adjustment or replacing If belts only squeak when damp, they probably just need adjustment. If they squeak constantly, they probably need to be replaced Exhaust heat shield noises Rattle that you hear when car is idling Damaged heat shield Replace heat shield

Common Car Noises and Their Likely Causes Noise More Details Likely Cause Fixing It Clicking noise while turning Starts faint, gradually becomes more noticeable until very loud Failed outer CV joint (constant velocity joint) due to torn boot (most likely) or old age Replace either the whole axle assembly (recommended) or the outer CV joint Water pump noises Difficult to diagnose because of water pump's location Noise usually caused by damage to the bearings in failed water pump Only cure is replacing it. If possible do it at the same time as a timing belt replacement to cut down on labor costs Squeak or squeal from an engine belt Loud, high-pitched squeak or squeal Belts either need adjustment or replacing If belts only squeak when damp, they probably just need adjustment. If they squeak constantly, they probably need to be replaced Exhaust heat shield noises Rattle that you hear when car is idling Damaged heat shield Replace heat sh

it depends what kind of noise, if its a licking noise the pulleys that the belt is being turned by could be bad (belt tensioner pulley ,Idler pulley etc.) if its a squealing sound like a high pitched whistle or like a rubber heal screeching on a tile floor constantly the belt is no good there is fabric on the inside of the belt and when it wears away you will get the chirp or squealing /screeching sound from this. inspection of that belt will tell you look at the grooves if looks like shiny rubber you found it if it doesn't and the fabric is there then a pulley is the problem bad bearing can make the belt slip resulting in the squealing good luck LATER MAN!

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.

A squeal is almost ALWAYs a bad tensioner, worn idler, or a worn belt. Read this post to determine which

http://ricksfreeautorepairadvice.com/fix-serpentine-belt-noise/
Fix serpentine belt noise Ricks Free Auto Repair Advice

Possible solution may be a new serpentine belt. When you turn the steering wheel it is an added torque/tension against the belt from the power steering drive pulley. A worn and dry belt may tend to squeal as the extra grip on that pulley is applied. You would have to pinpoint just where the squeal is coming from to be sure the power steering pump is the origin of the noise. There may also be other reasons if its due to a worn out belt, idler and tensioner pulley's could be making noise as well.

is it comming from the serp belt?look when the car is off to see if the belt looks brown and shiny and or cracked,,it could be the belt is loose or just old and worn and slipping on the pulleys,,may be your noise,or if you mean squealing while driving it could be the brake pads are low and the low indicator squealers are hitting the rotors to let you know the pads are low and need replacement

You need to tighten/replace your alternator belt as it is either worn or loose.

The noise is most likely the serpentine belt. Check it's condition and that of the belt tensioner. It sounds like the belt is slipping, causing the squealing noise u describe. As the belt warms up, it's getting a better 'grip' and the noise stops. Turning on the heat also turns on the a/c in newer cars, this added drag on the belt (one more component to turn, the a/c compressor) would explain why it happens when u turn on the heat. Is the belt original? If yes, it's time to change it, and check the belt tensioner to make sure it's doing it's job of keeping the belt snug on all the pulleys. Let me know how u make out.