Noise for air Conditioning

Noise for air Conditioning

Whenever I turn my AC on in my car every now and then i hear this like high pitched squeeking noise and then it will go away. Most of the time if I only have it blowing on the first setting it rarely happens but when I put it on the second fan setting it does it a lot. When the heat it on it never happens. Anyone have any ideas, I know somewhere there is a small leak because over the past year the coolness of the air blowing out has gotten warmer. It is a very small leak though because my mechanic put a dye in the coolant the R134 and after a day could not find anything so I doubt the leak is making the noise since the noise isnt constant. Maybe a valve or something any help is appreciated. its a 2001 corolla

The noise is inside the cabin and makes it whenever the air is on weather it is just the fan or the A/C it makes the squeeking noise