Hmmmm.....having drained the system, there MUST be some air in there to be displaced but - I'd have thought most of it would go forward to the carbs. I guess some could be trapped in the tank gubbins.
  As long as this bubbling does not continue I would not worry too much. 
 Puts on his best Macho voice " Just go out and thrash it for a while Marra"!!!! 
  Frank