The Collaborative International Dictionary of English v.0.48:

Humming \Hum"ming\, a.
   Emitting a murmuring sound; droning; murmuring; buzzing.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Humming \Hum"ming\, n.
   A sound like that made by bees; a low, murmuring sound; a
   hum.
   [1913 Webster]

   Hummingale, lively or strong ale. --Dryden.

   Humming-bird moth (Zool.), a hawk moth. See Hawk moth,
      under Hawk, the bird.
      [1913 Webster] hummingbird

The Collaborative International Dictionary of English v.0.48:

Hum \Hum\ (h[u^]m), v. i. [imp. & p. p. Hummed; p. pr. & vb.
   n. Humming.] [Of imitative origin; cf. G. hummen, D.
   hommelen. [root]15.]
   1. To make a low, prolonged sound, like that of a bee in
      flight; to drone; to murmur; to buzz; as, a top hums. --P.
      Fletcher.
      [1913 Webster]

            Still humming on, their drowsy course they keep.
                                                  --Pope.
      [1913 Webster]

   2. To make a nasal sound, like that of the letter m
      prolonged, without opening the mouth, or articulating; to
      mumble in monotonous undertone; to drone.
      [1913 Webster]

            The cloudy messenger turns me his back,
            And hums.                             --Shak.
      [1913 Webster]

   3. [Cf. Hum, interj.] To make an inarticulate sound, like
      h'm, through the nose in the process of speaking, from
      embarrassment or a affectation; to hem.
      [1913 Webster]

   4. To express satisfaction by a humming noise.
      [1913 Webster]

            Here the spectators hummed.           --Trial of the
                                                  Regicides.
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   Note: Formerly the habit of audiences was to express
         gratification by humming and displeasure by hissing.
         [1913 Webster]

   5. To have the sensation of a humming noise; as, my head
      hums, -- a pathological condition.
      [1913 Webster]

WordNet (r) 3.0 (2006):

humming
    n 1: a humming noise; "the hum of distant traffic" [syn: hum,
         humming]
    2: the act of singing with closed lips

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "humming":
   bel canto, birring, bombilation, bombinating, bombination, booming,
   bravura, burring, busy, buzzing, choral singing, coloratura, croon,
   crooning, droning, folk singing, fussy, hopping, hum, hustling,
   intonation, lively, low rumbling, lyricism, operatic singing,
   popping, purring, scat, scat singing, singing, sol-fa,
   sol-fa exercise, solfeggio, solmization, song, thrumming,
   tonic sol-fa, vocal music, vocalization, warbling, whirring,
   whizzing, yodel, yodeling