It is preferable to reduce the combustion noise of residential gas appliances from the view point of keeping the quiet living environment. To understand the generation mechanism of the combustion noise from a residential gas boiler, noise measurements and CFD analyses were carried out. As a result of those analyses, it was clarified that an oscillation arose in the combustion air flow at the inlet of the gas-air mixing pipe and the peak frequency of the oscillation coincided with the frequency of the combustion noise. This coincident oscillation was considered to amplify the combustion noise. It was found that the combustion noise reduction was achieved under the condition of controlling the coincident oscillation.