It's not clear what you mean by "ac unit," but if you mean it's noisy inside the car, sort of a moaning sound, with only limited air coming from the vents, then this is familiar to me. We had this problem on our 99 S70, intermittent, only happens on a long drive. In our case, we had the climate control set to "Auto," and that was the problem. We now control everything manually and we've not experienced the problem since. I suspect a damper somewhere is sticking, and this would be a frustrating thing to diagnose because it is intermittent.