GZDeamon

Alright my mustang has a pretty nasty squeal too it whenever I turn my ac on. When nothing is on it doesn't squeal what so ever. But if I turn my ac on while the car is on it will start to squeal after a few minutes, don't know how long it does it because when I hear it I turn the car off turn the ac off then start the car up again and it's fine.

I can turn my heat on while the car is still on and it works as it should. But if the car is off and the ac or heat is on and I start up the car it will squeal.

I had a problem awhile ago where the car squealed with or without the ac on. I had the belts replaced, idler pully, water pump and the tensioner pully all replaced. I'm leaning towards the ac compressor it's self. But I'd like to know what else this could be caused by before replacing the ac compressor

WJBertrand

Depending on your mileage, the accessory belt may be worn out and slipping making the squeal when the extra load of the AC is added. I could also be a bad serpentine belt tensioner not keeping enough tension on the belt, again allowing it to slip. Finally, your AC compressor may be going bad. I'd start with the cheapest, easiest thing and replace the belt.