The Collaborative International Dictionary of English v.0.48:

Ruba-dub \Rub"a-dub\, n.
   The sound of a drum when continuously beaten; hence, a
   clamorous, repeated sound; a clatter.
   [1913 Webster]

         The rubadub of the abolition presses.    --D. Webster.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rub-a-dub
    n 1: the sound made by beating a drum [syn: rub-a-dub,
         rataplan, drumbeat]