If the offset is fixed, the effect is called a comb filter. If the offset is changing, the effect is called flanging. The name stems from recording engineers rubbing their fingers against the flange of a reel-to-reel recorder's tape reel, to brake it slightly, which adds increasing delay to the sound.

Huh so a flange is just a combing effect that shifts around? I'd never considered this. Also neat fact about flanges, didn't know that either. Thanks!