  The origin of quasi periodic modulations of flux in the kilohertz range (kHz
QPOs), observed in low-mass X-ray binaries, is usually assumed to be physically
distinct from that of the ``normal branch oscillations'' (NBOs) in the
Z-sources. We show that a low-frequency modulation of the kHz QPOs is a natural
consequence of the non-linear relativistic resonance suggested previously to
explain the properties of the high-frequency twin peaks. The theoretical
results discussed here are reminiscent of the 6 Hz variations of frequency and
amplitude of the kHz QPOs reported by Yu, van der Klis and Jonker (2001).
