I have factory installed AC and it made this noise all the time when it was on either defog, or defog and foot vent settings (but not on any other). I brought it in to get it fixed under warranty and they replaced the blower motor and claimed that the fan blades were bent. It stopped for awhile but seems to have started again though it does not occur all the time and isn't nearly as loud. Seems to me like there is some other underlying issue that is causing this noise. I will try to get an audio clip of mine to see if it is the same issue/so you understand what we mean.