The Collaborative International Dictionary of English v.0.48:

Drone \Drone\ (dr[=o]n), v. i. [imp. & p. p. Droned; p. pr. &
   vb. n. Droning.] [Cf. (for sense 1) D. dreunen, G.
   dr["o]hnen, Icel. drynja to roar, drynr a roaring, Sw.
   dr["o]na to bellow, drone, Dan. dr["o]ne, Goth. drunjus
   sound, Gr. ? dirge, ? to cry aloud, Skr. dhran to sound. Cf.
   Drone, n.]
   1. To utter or make a low, dull, monotonous, humming or
      murmuring sound.
      [1913 Webster]

            Where the beetle wheels his droning flight. --T.
                                                  Gray.
      [1913 Webster]

   2. To love in idleness; to do nothing. "Race of droning
      kings." --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

droning
    n 1: an unchanging intonation [syn: monotone, drone,
         droning]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "droning":
   birring, bombilation, bombinating, bombination, booming, burring,
   buzzing, drone, gabble, gibber, gibbering, hum, humming, jabber,
   jibber, low rumbling, maundering, monotone, monotonic, mouthing,
   mumble, mumbling, murmuring, mutter, muttering, purring,
   soniferous, sonorous, sounded, sounding, susurration, thrumming,
   tonal, toneless, whirring, whisper, whispering, whizzing