The Collaborative International Dictionary of English v.0.48:

bum \bum\, v. i. [imp. & p. p. bummed; p. pr. & vb. n.
   bumming (?).] [See Boom, v. i., to roar.]
   To make murmuring or humming sound. --Jamieson.
   [1913 Webster]

   to bum around to wander about idly or aimlessly.
      [PJC]