Time-dependent microwave absorption in Bi-system crystals manifests itself as modulation of the cavity Q-factor with a frequency of about 1 Hz. We studied the dependences of the oscillation amplitude and frequency on the temperature, microwave power, DC magnetic field H and its modulation amplitude. The maximal amplitude of low-frequency oscillations was observed at H = 0, and it did not depend on temperature in the range 3.5-60 K, the critical temperature being T_c = 84 K. The effect is supposed to arise from self-arrangement of inner Josephson junctions via the microwave field.