  Optically pumped magnetometers (OPMs) are revolutionising the task of
magnetic-field sensing due to their extremely high sensitivity combined with
technological improvements in miniaturisation which have led to compact and
portable devices. OPMs can be based on spin-oriented or spin-aligned atomic
ensembles which are spin-polarized through optical pumping with circular or
linear polarized light, respectively. Characterisation of OPMs and the
dynamical properties of their noise is important for applications in real-time
sensing tasks. In our work, we experimentally perform spin noise spectroscopy
of an alignment-based magnetometer. Moreover, we propose a stochastic model
that predicts the noise power spectra exhibited by the device when, apart from
the strong magnetic field responsible for the Larmor precession of the spin,
white noise is applied in the perpendicular direction aligned with the
pumping-probing beam. By varying the strength of the noise applied as well as
the linear-polarisation angle of incoming light, we verify the model to
accurately predict the heights of the Larmor-induced spectral peaks and their
corresponding line-widths. Our work paves the way for alignment-based
magnetometers to become operational in real-time sensing tasks.
