  We report on the X-ray aperiodic timing analysis of two accreting millisecond
pulsars: XTE J1807-294 and IGR J00291+5934. On the one hand, we discovered in
XTE J1807-294 seven pairs of simultaneous kilohertz quasi-periodic oscillations
(kHz QPOs) separated in frequency by nearly the spin frequency of the neutron
star. This confirms the suspected dichotomy in the frequency separation of kHz
QPOs: sometimes once and sometimes half the spin frequency. On the other hand,
we found an extreme behavior in the power spectra of IGR J00291+5934: very
strong variability at very low frequencies. Namely, the fractional amplitude of
the variability was ~50%, the highest value found so far in a neutron star
system.
