Performed preventive maintenance to furnace. Checked for proper and safe operation, maximum efficiency, and any abnormalities. Replaced the filters, cleaned the flame sensor, checked the blower motor and capacitor, checked the igniter, checked the inducer motor. Checked the thermostat operation in heating mode. Note: the furnace is making a noise on top of furnace where the fluid connects. Unable to disconnect the flue at this time to see where the noise is coming from. The noise becomes less as the furnace stays on longer. The noise seems to not affect in anyway the performance of the furnace. If noise continues and customer wants further inspection of it, the cost to do so is at time and material.