In recent years, environmental noise has become a major concern in large urban areas, significantly reducing the quality of life and health. The measurement, localization and monitoring of various environmental noise sources are the key elements of success in tackling the urban noise problem. However, the deployment of fixed microphones at a dense grid of locations becomes prohibitively expensive for city-wide sound pressure level (SPL) measurements over long periods of time. Instead, we propose a novel low-cost large-scale environmental noise mapping approach based on vehicles with rooftop-mounted microphone arrays driving around the city. As the vehicle drives past, the locations and SPLs of sparsely-distributed localized noise-sources in the acoustic field are determined via tomographic noise-maps, in which the delay-and-sum beamforming power outputs of the acoustic data collected at multiple locations and different times are considered as tomographic measurements. The drive-by acoustic noise-source maps produced from the preliminary experiments with two circular arrays mounted on an electric vehicle demonstrate the promise of this new approach for city-scale environmental noise monitoring to identify the regular environmental noise patterns cost-effectively.