This is going to sound crazy. About a month ago my wife was driving the GXP. She stopped at Walmart, did her shopping, and started the car and heard a noise. Called me, I was afraid a belt idler or tension bearing was going out. I drove there had her start the car and all sounded good. So we swap cars and I started driving it. No problems until this morning. I back out of the garage, started down the driveway when I heard the whine. My first thought was belt tension bearing, but then it sounded closer to the front of the engine. So I killed the motor and the noise did not stop with the rotation of the engine. In fact, it was more of a winding down. Then I started the car and drove it a couple of places and so far it has not done it again. Of course, it has been almost a month since the last time it did it.
Thinking about it, I wonder if it could be a smog pump?
