I have something similar...it only happens when the tank is empty. If you open the fuel cap the noise disappears. So based on that I suspect either the cap is not sealing properly or there is a directional pressure release value that is not working properly. One day I'll apply my mind to it.
Come to think of it there is a massive rush of air when the fuel cap comes off. Maybe there's a partially blocked breather somewhere. Does anyone know where that might be? In the fuel cap itself perhaps?