My car squeals when I turn on the AC. The belt is pretty worn, or what I think is the belt. It use to blow a little bit of cool air until the squealing started. Now it smells like burnt rubber and hot air.

v belts -- and am assuming that is what you have -- have to in position just proud of the edge of the pulley to work well
if the back of the belt is below the edge of the pulley it is worn or the pulleys are worn
the belts come in different cross sections and a smaller belt in section in a wider pulley will run on the bottom of the pulley and will slip and squeal just like you describe
belts come in "V" section , "A" section and "B" sections

It sounds to me that your belt or tensioner is worn, if the noise goes away when the ac is off then it is just the belt slipping, it takes a lot of power to run the compressor and if the belt is worn it most likely slipping on the pully when you engage the clutch.

The noise is probably the compressor. Maybe the belt could be loose or worn. You may be low on freon. Start with the easy one, check the belt, then freon. Good luck!

So many mechanics, so little consensus. Sounds pretty common.
I can't say as to why hot and cold only when moving, but it sounds like a weak performing system.
As for the squealing, I would suggest removing one belt at a time, and see if one eliminates the squeal. That would be the problem belt or pulley or component. Also, if you have any idler pulleys (a stand-alone pulley) on the belt system, be sure and check them as well. Spin them without the belt on, and see if they make any noise.
It would seem you need a mechanic who is willing to spend some time with you on this to find the problem. When you find a good one, you will know it by his output, and make yourself a regular customer. Good luck.

check the AC belt

you need to replace the serpentine belt and perhaps the belt tensioner. When a heavy load is put on the belt, like turning on the AC, the drag may stop the belt from turning. This would stop the power steering pump from turning causing a hard steer condition. The belt is not gripping the pulleys tight enough. This is due to a worn out serpentine belt or worn out tensioner. If the AC works( cools the car) then replace the belt. If this does not cure the problem then replace the belt tensioner as well.

The squeeling is the belt because the air compresor is likely seizing. At idle it can't turn the compressor, in motionthe compressor probably turns and cools the air. Most air conditioners come on when the defroster is on.

yes...your engine belt is little bit lose that's y its causing noise...and hope when u speed up..it will stop squealing ..right?? so just tight your engine belt little bit and it will solve your problem in a min...take care n have a safe journey...n also if it help...rate it plz

When the engine is not running the belt is not turning the ac compressor which will not blow cold air unless ac pump is running so engine has to be running

If your refrigerant level is OK and the truck blows cold air your compressor should be fine, in which case the noise is probably coming from the AC clutch which isn't too expensive or difficult to replace. If it doesn't blow cold air, switch the AC on and watch the compressor. If the clutch engages, but the compressor doesn't turn, the drive belt will squeal loudly and probably start to smoke. In that case it is the compressor which is expensive to replace. Most vehicles have an AC bypass that will temporarily cut out the compressor clutch under heavy load. If you are going uphill with the AC on or accelerating heavily you might notice warm air coming out of the vents for a short period until you let off the gas.