I had the same problem with my 98 S70. What was happening with my S70 was a switch was sticking. Perhaps on the high-pressure side? I can't remember. When you have your ac on it cycles on and off, when this switch goes bad the ac doesn't cycle off then the system freezes up and you hear the fan make that noise because the fan cannot push air through the evaporator. One way to make sure this is your problem is to cut your ac on and then listen to see if the clutch on the compressor cycles on and off, if it doesn't than the switch is the problem.