  We present an analysis of the X-ray variability of three symbiotic X-ray
binaries, GX 1+4, 4U 1700+24, and 4U 1954+31, using observations made with the
Swift Burst Alert Telescope (BAT) and the Rossi X-ray Timing Explorer (RXTE)
All-Sky Monitor (ASM). Observations of 4U 1954+31 with the Swift BAT show
modulation at a period near 5 hours. Models to explain this modulation are
discussed including the presence of an exceptionally slow X-ray pulsar in the
system and accretion instabilities. We conclude that the most likely
interpretation is that 4U 1954+31 contains one of the slowest known X-ray
pulsars. Unlike 4U 1954+31, neither GX 1+4 nor 4U 1700+24 show any evidence for
modulation on a timescale of hours. An analysis of the RXTE ASM light curves of
GX 1+4, 4U 1700+24, and 4U 1954+31 does not show the presence of periodic
modulation in any source, although there is considerable variability on long
timescales for all three sources. There is no modulation in GX 1+4 on either
the optical 1161 day orbital period or a previously reported 304 day X-ray
period. For 4U 1700+24 we do not confirm the 404 day X-ray period previously
proposed for this source from a shorter duration ASM light curve. We conclude
that all three sources have substantial low-frequency noise in their power
spectra that may give the appearance of periodic modulation if this noise is
not properly accounted for, particularly if short duration light curves are
examined.
