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