  The measurement of the spin frequency in accreting millisecond X-ray pulsars
(AMXPs) is strongly affected by the presence of an unmodeled component in the
pulse arrival times called 'timing noise'. We show that it is possible to
attribute much of this timing noise to a pulse phase offset that varies in
correlation with X-ray flux, such that noise in flux translates into timing
noise. This could explain many of the pulse frequency variations previously
interpreted in terms of true spin up or spin down, and would bias measured spin
frequencies. Spin frequencies improved under this hypothesis are reported for
six AMXPs. The effect would most easily be accounted for by an accretion rate
dependent hot spot location.
