Question about a midi file

[Patrick or Cynthia Karl]

When I compile the following:

 \version "2.19.15"

Drum  = \drummode { 
  \set Staff.midiInstrument =#"woodblock"
  hiwoodblock4 wbh8 wbh wbh wbh wbh4
  r4 r8 wbh8 r4 r8 wbh8
  wbh4 \repeat unfold 12  { wbh16  }
}

\score {
  \new DrumStaff \with { drumStyleTable = #percussion-style
                                          \override StaffSymbol.line-count = #1 
}
                                     \Drum
  \layout { }                
  \midi { }                                           
}

the midi notes appear on channel 10 as they should, but they are overlaid by 
what appears to be another percussion instrument playing the following 
simultaneously:

    \repeat unfold 3 { toml4\f toml\p toml toml }

i.e., it sounds like a 4-beat metronome.  Here I am using toml only because I 
don't know what percussion instrument is actually involved.

What might be the explanation for this?  and how to stop it?