  We present a simultaneous periodic and aperiodic timing study of the
accreting millisecond X-ray pulsar SAX J1808.4-3658. We analyze five outbursts
of the source and for the first time provide a full and systematic
investigation of the enigmatic phenomenon of the 1 Hz flares observed during
the final stages of some of the outbursts. We show that links between
pulsations and 1 Hz flares might exist, and suggest they are related with
hydrodynamic disk instabilities that are triggered close to the
disk-magnetosphere boundary layer when the system is entering the propeller
regime.
