quavering

(verb) give off unsteady sounds, alternating in amplitude or frequency. Synonyms: quaver, waver.

(verb) sing or play with trills, alternating with the half note above or below. Synonyms: warble, trill, quaver.

(adj) (of the voice) quivering as from weakness or fear. Synonyms: tremulous. Examples: "The old lady's quavering voice." "Spoke timidly in a tremulous voice."