  We perform spin noise spectroscopy experiments in metastable helium atoms at
room temperature, with a probe light whose frequency is blue detuned from the
D0 line. Both circular birefringence fluctuations (Faraday noise) and linear
birefringence fluctuations (ellipticity noise) are explored theoretically and
experimentally. In particular, it is shown that in both cases but for different
optical detunings, two noise resonances are isolated at the Larmor frequency
and at twice the Larmor frequency with a behaviour, which strongly depends on
the orientation of the probe field polarization. The simple structure of
metastable helium allows us to probe, model and explain the changes in the
behavior of these peaks in terms of circular and linear dichroisms and
birefringences as well as in terms of spin oscillation modes.
