If the furnace has just been started there may not be enough condensate built up in the furnace condensate trap to be effective. Pour some water into the trap and it should stop. the Manufacturers recommend this when we start up a furnace. Possible the trap has a crack in it that allows the water to seep out and again cause an ineffective trap for the condensate . The combustion vent blower pulls a negative pressure on the trap so there is little chance of flue gasses excaping. It can throw off the gas air mix to the burners however.