  In a search for periodic variation in the arrival times of pulses from 151
young, energetic pulsars, we have identified seven cases of modulation
consistent with one or two harmonics of a single fundamental with time-scale
0.5-1.5 yr. We use simulations to show that these modulations are statistically
significant and of high quality (sinusoidal) even when contaminated by the
strong stochastic timing noise common to young pulsars. Although planetary
companions could induce such modulation, the large implied masses and 2:1 mean
motion resonances challenge such an explanation. Instead, the modulation is
likely to be intrinsic to the pulsar, arising from quasi-periodic switching
between stable magnetospheric states, and we propose that precession of the
neutron star may regulate this switching.
