  We investigate rotational spin noise (referred to as timing noise) in
non-accreting pulsars: millisecond pulsars, canonical pulsars, and magnetars.
Particular attention is placed on quantifying the strength and non-stationarity
of timing noise in millisecond pulsars because the long-term stability of these
objects is required to detect nanohertz gravitational radiation. We show that a
single scaling law is sufficient to characterize timing noise in millisecond
and canonical pulsars while the same scaling law underestimates the levels of
timing noise in magnetars. The scaling law, along with a detailed study of the
millisecond pulsar B1937+21, leads us to conclude that timing noise is latent
in most millisecond pulsars and will be measurable in many objects when better
arrival time estimates are obtained over long data spans. The sensitivity of a
pulsar timing array to gravitational radiation is strongly affected by any
timing noise. We conclude that detection of proposed gravitational wave
backgrounds will require the analysis of more objects than previously suggested
over data spans that depend on the spectra of both the gravitational wave
background and of the timing noise. It is imperative to find additional
millisecond pulsars in current and future surveys in order to reduce the
effects of timing noise.
