In an applied research project on a pulsed microwave sulfur lamp prototype of 1 kW, fitted with a rotation less and electrode less spherical bulb, we discovered that the plasma may form, despite gravity, a ball of about half the bulb size, settled in the center. In a preceding publication, we then reported measurements performed with a photodiode that shows the high-pressure plasma response to short microwave pulses, and we showed by modelization that the ball formation results from an acoustic resonance in a spherical mode. Out of this formation, the signal AC component has the same frequency as the pulse rate, and resembles to a triangular signal, rising during the ON periods and falling back during the OFF periods. When the ball formation occurs, at a pulse rate a little below 30 kHz, the AC component changes to a sinusoidal signal of a slightly lower frequency, and beats appear with a frequency equal to the frequency shift. In the preceding publication, it was demonstrated that the beats could result from the simultaneous excitation of two normal modes, because they have a frequency difference matching the observed frequency shift. As the higher of the two frequencies is the pulse rate, the one is due to a forced oscillation, whereas the other one is due to a free oscillation. In this article, we study the dissipation due to bulk viscosity and, thus, identify a mechanism that can couple the two oscillations, explaining the simultaneous excitation.