According to recent investigations, the neutron noise field in a boiling water reactor can be separated into a local and a global component. In the present paper, these two components are discussed further via two-group diffusion theory. The expediency of the local-global concept is compared to another concept based on separating components corresponding to the two roots of the dynamic eigenvalue problem. The mathematical discussion of the neutron response to a propagating perturbation of the moderator density is given. Point reactor behavior and “linear-phase behavior” appear as two extremes of the neutron response. The mathematical results are illustrated numerically for the cases of a large power reactor core and a small highly enriched core.