I don't know for sure. I'm not a physicist. I would assume it could be the metal in the plane, vortices created by its motion through the air, changes to the Ea

I don't know for sure. I'm not a physicist. I would assume it could be the metal in the plane, vortices created by its motion through the air, changes to the Earth's magnetic field caused by the metallic object moving through it, something like that.