  We present a timing and noise analysis of the Be/X-ray binary system Swift
J0243.6+6124 during its 2017-2018 super-Eddington outburst using NICER/XTI
observations. We apply a synthetic pulse timing analysis to enrich the
Fermi/GBM spin frequency history of the source with the new measurements from
NICER/XTI. We show that the pulse profiles switch from double-peaked to
single-peaked when the X-ray luminosity drops below $\sim$$7\times 10^{36}$ erg
s$^{-1}$. We suggest that this transitional luminosity is associated with the
transition from a pencil beam pattern to a hybrid beam pattern when the Coulomb
interactions become ineffective to decelerate the accretion flow, which implies
a dipolar magnetic field strength of $\sim$$5\times 10^{12}$ G. We also
obtained the power density spectra (PDS) of the spin frequency derivative
fluctuations. The red noise component of the PDS is found to be steeper
($\omega^{-3.36}$) than the other transient accreting sources. We find
significantly high noise strength estimates above the super-Eddington
luminosity levels, which may arise from the torque fluctuations due to
interactions with the quadrupole fields at such levels.
