undercurrent

(ŭnˈdər-kûrˌənt, -kŭrˌ-)

1. A current, as of air or water, below another current or beneath a surface.
2. An underlying tendency, force, or influence often contrary to what is superficially evident; an intimation: “The Gaucho began to talk, calmly but with an undercurrent of passion” (Thomas Pynchon).