The Collaborative International Dictionary of English v.0.48:

Warble \War"ble\, v. t. [imp. & p. p. Warbled; p. pr. & vb. n.
   Warbling.] [OE. werbelen, OF. werbler; of Teutonic origin;
   cf. G. wirbeln to turn, to warble, D. wervelen, akin to E.
   whirl. See Whirl.]
   [1913 Webster]
   1. To sing in a trilling, quavering, or vibratory manner; to
      modulate with turns or variations; to trill; as, certain
      birds are remarkable for warbling their songs.
      [1913 Webster]

   2. To utter musically; to modulate; to carol.
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            If she be right invoked in warbled song. --Milton.
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            Warbling sweet the nuptial lay.       --Trumbull.
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   3. To cause to quaver or vibrate. "And touch the warbled
      string." --Milton.
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Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "warbling":
   bel canto, bravura, choral singing, coloratura, croon, crooning,
   folk singing, hum, humming, intonation, lyricism, operatic singing,
   scat, scat singing, singing, sol-fa, sol-fa exercise, solfeggio,
   solmization, song, tonic sol-fa, vocal music, vocalization, yodel,
   yodeling