ok .. Have you left your car unused for extended periods of time ? If so please ask the service centre to check the valves and the injectors . Ask the service guys to check if there is a difference in the engine noise . Even for brand new cars the cars are in storage for extended periods of time ,especially Fiat cars due to low sales . Also try disconnecting the air mass sensor and drive the car . Let us know if you find any difference in mileage . When you disconnect the air mass sensor you might get a warning or an error , just ignore it .