Air Compressor for 2001 Plymouth Voyager?

My van has been making a loud squeaking noise when I have been driving it for about a year now. Recently the squeaking has gotten louder and I took it to the shop. I was told that the noise was coming from my air compressor and that I needed to be replaced? He tested the car without the ac running and no squeak. He said the car would be fine as long as I didn't run the ac. If I did run the ac, eventually the air compressor would break. If the air compressor is the source of the squeak, can I run the air without the air conditioner on and have the car not squeak? Does the air compressor only run when the ac is on or also when just the air is blowing or also with the heater?