Thanks oldskewel for all that helpful info. I just took a long screwdriver out to my car and can confirm that the high pitched whirring noise is definitely coming from the alternator. I'm leaning towards replacing with an OEM Valeo 140A alternator (which is what's in the car now) from FCP. The noise doesn't really bother me, but I need to drive my car from NY to Chicago and back over the next few weeks. Should I look to do the replacement before I leave for the trip? Or...I could get the new alternator and throw it in the back of the X just in case I run into trouble along the way.