We analyzed the 17 August 1998 zebra event and showed that some quasi-periodic oscillations modulate the zebra-stripe frequencies. We determined the period of these oscillations as P _n= 2.01 ± 0.03 (in numbers of zebra stripes) and as P _f= 11.8 ± 0.17 MHz. In the first part of the analyzed zebra, we found a stable density wave that slowly propagated with the frequency drift less than 0.4 MHz s ⁻¹. Then, a stationary density wave appeared followed by a transformation of the waves to ones with longer periods. These long-period waves were recorded before and after the time interval when no zebra stripes were observed. We interpreted these density waves as magnetosonic waves. We calculated their wavelength and propagating velocity, considering two types of density models of the solar atmosphere. We also estimated the characteristic density and magnetic-field strength as N≈ 9.2 × 10 ⁸ cm ⁻¹ and B≈0.73G, respectively. We found similar velocities derived from drifts of the density wave and velocities calculated from the density and magnetic-field strength considering gyro-harmonic numbers of zebra stripes.