I noticed you are in Wisconsin. Heres the thing, I have an '01 also, and I know EXACTLY what whistling noise you are talking about. I used to live there up until about 2 1/2 years ago. The whistling on mine only came around when the cold weather showed up. In the summer, GONE! I never did figure out what it was, but after I moved here to Arizona, I have never heard it again! Aside from that, I had that code for quite a while, always seemed to come back even after changing the gas cap. Finally found that the little rubber elbow that attaches a vacuum line to the outside surface of the intake manifold was deteriorated. It looked fine at first glance, but as soon as I bent it and inspected it, all the cracks showed up and even a small rotted out hole in it. Changed it out and the code never came back. So just give all those little rubber vacuum fittings a check. Good luck on figuring out the whistle. Or just move to a warm climate to solve that issue!