94 Subaru Impreza, manual transmission. When I turn the fan on, it makes sort of a chugging wheezing sound like someone having trouble breathing. Not very loud, but noticeable and you can feel the engine struggle just a bit every time it "exhales." This is only when its idling. Once placed in gear and moving, it subsides until it goes away completely at highway speeds. If I idle long enough, it sometimes stalls. Does it with AC, heat, or just the fan. No symptoms of engine trouble at all when the fan is off. No check lights on. Started doing it right after my last oil change.