The acoustic behavior of a double annular resonator is studied. Compared to the associated single resonator, the double resonator shows a transmission peak near the resonance frequency where the transmission is minimal when the losses are zero. This peak is associated with an evanescent coupling between the two resonators and is called "Autler-Townes splitting". During the measurement of the device, this peak is very strongly attenuated by viscous effects, so much so that it almost disappears. When a flow is added, even a very small one, a gain is created and the peak reappears at the resonator frequency. As the mean flow velocity increases, this gain can become large enough for a whistling sound to appear. For this velocity of appearance of the whistling, the effect of the flow on the single resonator is negligible.