Grrrr....Woo-Woo Noise From Fuel Filler Door is Back.

[Need_for_speed]

Thought I had this one solved.

It started with a steady "blowing over a bottle" noise at around 3,000 rpm. Any change in throttle would make it go away. No CEL or codes. After a lot of research, I replaced the two valves in the front passenger side fender area behind the headlight. No effect.

So then I decided to replace the regen valve under the intake manifold. After a few test drives, I thought I had the problem solved.

Unfortunately, on my last few drives, the noise is back, but in a different way. I only hear it very intermittently -- lasts for only 2-3 seconds. The noise is not responsive to throttle position and occurs at just about any rpm.

It's definitely coming from the fuel filler area, but before I start throwing even more money into parts, I'm wondering what the next best guess for the cause might be. I have yet to replace the valve on the filler neck. I understand there is also a valve on the carbon canister. I've looked at the carbon canister -- don't see any cracks or anything unusual. Any ideas would be appreciated. Thanks!

Edit: forgot to add that I also replaced the fuel fill cap.