It may be coming from the gas fill area. There are two valves in the evap/charcoal cannister system that can go bad. They went out in my Boxster and it sounded like a howling/trumpet sound, came and went, RPM did not matter, and one time even did it at idle. If you look at a diagram of the gas fill/evap area you will see where they are located. Not are hard fix. Just a pain to get to. Hope this helps.
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