  We have investigated the stability of the pulse frequency second derivatives
($\ddot \nu $) of PSR 0823+26, PSR 1706-16, PSR 1749-28, PSR 2021+51 which show
significant quadratic trends in their pulse frequency histories in order to
determine whether the observed second derivatives are secular or they arise as
part of noise processes. We have used TOA data extending to more than three
decades which are the longest time spans ever taken into account in pulse
timing analyses. We investigated the stability of pulse frequency second
derivative in the framework of low resolution noise power spectra (Deeter 1984)
estimated from the residuals of pulse frequency and TOA data. We have found
that the $\ddot \nu $ terms of these sources arise from the red torque noise in
the fluctuations of pulse frequency derivatives which may originate from the
external torques from the magnetosphere of pulsar.
