Update on this wooooo sound. I replaced a valve block today that's right under the filler opening behind the fender liner. I don't really understand the vent system and I just guessed when I ordered this part. I was getting some other parts from a place that has a $75 minimum order requirement and needed to pad my purchase to make the minimum so I took a risk and bought this part. Actually this is the old one.
The upshot is that replacing this valve didn't fix the problem, so save your money and don't bother doing what I did. I played around with the old valve and discovered the functionality... it permits bidirectional air flow between the two ports whenever the gas cap is tightened. With the gas cap off it will still flow in one direction only. But my car still goes "woooooooo" under light throttle pretty often.
So if anybody else develops the known fix, I'm interested.