Noise coming from gas filler area.

[Rich XTR]

I believe there are 2 emission control valves that vent to the whole under the gas filler door.When i replaced mine i was worried that the one i replaced might be the wrong one....but it fixed it. in your case you might have not replaced the one causing the noise. the issue is exactly as you describe. blowing over a coke bottle sound coming out of the whole under the gas filler door.

[pomerlo]

If you have a Durametric and want to test the valves see this post #6 and #7

http://986forum.com/forums/boxster-general-discussions/75684-hard-restart-after-fueling.html

I temporarly fixed the issue on mine by putting a small piece of masking tape to partially cover the vent hole(half of it). The one located a 3 oclock in the housing.
A foam would work as well !