The Collaborative International Dictionary of English v.0.48:

Twang \Twang\, v. i. [imp. & p. p. Twanged; p. pr. & vb. n.
   Twanging.] [Of imitative origin; cf. Tang a sharp sound,
   Tinkle.]
   To sound with a quick, harsh noise; to make the sound of a
   tense string pulled and suddenly let go; as, the bowstring
   twanged.
   [1913 Webster]