96 Windstar Squeals

My 96 Windstar has developed a squealing noise whenever I move the air control knob anywhere except off.It starts within 30 seconds or so. If I turn the air control knob off the noise qoes away within 15 seconds or so. I had my wife shut off the van while I listened for the noise. When the engine died I still heard the noise for a couple of seconds. All the belts and every other visible moving parts had stopped. Could it be the AC compressor going bad. The air is nice and cold.