Exterior noise level measurements were conducted on a typical in-service diesel engine heavy duty truck to determine the noise level reduction resulting from a turbocharger retrofit. The tests were conducted in accordance with procedures recommended by the Society of Automotive Engineers and the United States Department of Transportation. Overall noise level measurements and a narrow-band spectrum analysis indicate that a significant noise level reduction is obtained from the installation of a turbocharger kit. The lower noise level is the result of a smoother pressure development during combustion. The narrow-band analysis shows a significant amplitude reduction at the engine firing frequency and a general decrease in noise energy over the whole frequency range.