This morning I turned the key to the on position. The windscreen goes up and the fuel pump hums to life and when all that's done I hear a noise like a small fan running. I didn't notice it yesterday, but I had the radio off today. It sounds like it's coming from in or around the fuel tank, but it's too faint to be the fuel pump and it sounds like a fan (it's not the radiator fan, engine was cold and I checked).
Is this normal and if so, what is making the noise?