Balloon engraver and annotations

[Andrew Bernard]

I just want an annotation to a note with a line pointing to it.
Thinking to use the balloon engraver, the example from the NR draws an
ugly line next to the note, presumably a degenerate box:

\new Voice \with { \consists "Balloon_engraver" }
{
  d'4
  \balloonGrobText #'Stem #'(3 . 4) \markup { "I'm a Stem" }
  d'4
}

How can the box be removed? Or more to the point, how can I do an
annotation like that, not a footnote callout?

Andrew