Lewis & Short

frĕmor, ōris, m. [fremo], a low roaring, rushing, murmuring (poet. and in postclass. prose for fremitus): arma sonant, fremor oritur, Poët. ap. Varr. L. L. 6, § 67 Müll.: variusque per ora cucurrit Ausonidūm fremor, Verg. A. 11, 297; Arn. 1, 32.
In plur.: leonum indignati fremores, App. Flor. p. 358, 4.