Synonyms for Roar (same or very similar meaning)
WordNet sense 1 (a deep prolonged loud noise):
boom, roaring, thunderWordNet sense 2 (a very loud utterance (like the sound of an animal)):
bellow, holler, yowl, bellowing, holla, hollering, hollo, holloaWordNet sense 3 (emit long loud cries):
howl, ululate, wail, yawl, yaupFrom the ODE community, based on WordNetadd/edit