Source : WordNet®

chug
     n : the dull explosive noise made by an engine
     v : make a dull, explosive sound; "the engine chugged down the
         street"
     [also: {chugging}, {chugged}]

Source : Free On-Line Dictionary of Computing

chug
     
         To run slowly; to {grind} or {grovel}.  "The disk is
        chugging like crazy."
     
        [{Jargon File}]
     
        (1995-03-25)