The Collaborative International Dictionary of English v.0.48:

Croon \Croon\, v. t. [imp. & p. p. Crooned (kr[=oo]nd); p. pr.
   & vb. n. Crooning.]
   1. To sing in a low tone, as if to one's self; to hum.
      [1913 Webster]

            Hearing such stanzas crooned in her praise. --C.
                                                  Bront['e].
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   2. To soothe by singing softly.
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            The fragment of the childish hymn with which he sung
            and crooned himself asleep.           --Dickens.
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