I experienced that with my unit. The furnace normally generates condensate during operation so it is important the condensate drain lines be open. The furnace will gurgle if the line is plugged and the furnace inducer fills up with condensate. Related to this is the need for the furnace to be installed plumb. If the furnace leans away from the condensate drain it will also build up condensate in the inducer cavity. That was the case with my furnace.