What does the random "ding" in Google Maps Navigation signify?
It sounds like you might be getting the "ding" noise discussed on this Google groups post:


  Thanks for the question. The alert chime you here indicates is a
  prompt for an upcoming driving maneuver. It plays in places where the
  Text-to-Speech engine is either unsupported or unavailable, but it
  also may play if the data connection becomes flaky during guidance and
  Navigation isn't able to process a road name in time.