verb

• (transitive) To modulate a tone's frequency.
• (transitive) To sing like a bird, especially with trills.
  "Her voice more sweet than warbling sound, Tho’ sung by nightingale or lark, Her eyes such lustre dart around, Compar’d to them the sun is dark."
• (transitive) To cause to quaver or vibrate.
  "touch the warbled string"
• (intransitive) To be quavered or modulated; to be uttered melodiously.
  "The wailings of a maiden I recite, / A maiden fair, that Sparabella hight. / Such ſtrains ne'er warble in the linnet's throat, / Nor the gay goldfinch chaunts ſo ſweet a note, [...]"

noun

• (military) In naval mine warfare, the process of varying the frequency of sound produced by a narrowband noisemaker to ensure that the frequency to which the mine will respond is covered.