Hi Tim,
Thanks for you thoughts. Unfortunately I'm pretty sure the noise is not something "normal" because:
it only started about a month agoit is really noticeable when driving at speeds under about 70 km/h (far too loud to be acceptable in a modern car).
It seems to occur regardless of how much fuel is in the tank.
I don't know if the noise happens when the fuel cap is removed. I tried removing the cap a couple of times and the noise stopped, but this may just be a coincidence as the noise is really intermittent - sometimes it occurs every 5-10 seconds, sometimes if may only occur every 5-10 minutes.
When inside the car, the noise is like a humming. When standing outside the car with the fuel filler "door" (not the cap) open, the noise is like a buzzing or vibrating sound (quite loud actually).