In this paper we study the mechanism of jet noise production in coaxial flows. We shall present various experimental results for isothermal and heated coplanar jet flows of four area ratios (0.81, 2, 3, and 4), operating at different velocity ratios, ranging from 0.60 up to 1.26. A polar array technique has been used to find the axial distribution of the jet sources. In order to better understand the behavior of the experimental results, some computational fluid dynamics simulations are performed, and a basic theoretical model based on Lighthill's Acoustic Analogy is also developed. It is shown that some significant changes occur to the flow structure and therefore the jet noise source distribution at velocity ratio of about 0.8, and also for the nozzles with area ratio of 2 to 4. It has also been shown that the most important high and low frequency sources for low velocity ratio flows are aggregated in a region about seven to ten secondary diameters downstream, while at higher velocity ratios sources are continuously spread from about one up to ten secondary diameters downstream. The effect of the primary flow temperature is also examined.