Recoil, torque and internal stock wood bearing on metal parts. Browning O/Us, which I owned and shot in competition for many years, (mostly 12 gauges), would exhibit this identical phenomenon. In the case of the Browning and other O/Us, FN gunsmiths claimed that because of the angle of the firing pins, one or the other (over or under) pins within their springs would have adequate room to vibrate the springs upon detonation, causing the wave of vibration to travel to the barrels and "release", so to speak, upon the vent rib or along the forend iron.
This produced the familiar "singing sound" heard upon shooting, which lingered for a few hundredths of a second after firing. The theory being that either one side or the other of the firing pin race and spring alignment of a SxS, or the upper or lower barrel of an O/U, was "tight" or "loose" in terms of transmitting the tuning-fork-type sound as described.
Particularly rapid firing in succession, as in skeet doubles or in bunker trap, would cause a dampening effect on the first detonation as the gun recoiled, thus attenuating the vibration upon the first shot, but allowing a lingering tone on the second shot, clearly audible as a "ringing" sound".
According to harmonic physics, the higher the quality of the steel, the more "finely tuned" was the ringing sound perceived, due to the more even transmission of the sound waves along the straighter lines of laminar flow of the higher quality steel.
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