  Formalism to calculate the hydrodynamic fluctuations by applying the Onsager
theory to the relativistic Navier-Stokes equation is already known. In this
work, we calculate hydrodynamic-fluctuations within the framework of the second
order hydrodynamics of M\"{u}ller, Israel and Stewart and its generalization to
the third order. We have also calculated the fluctuations for several other
causal hydrodynamical equations. We show that the form for the
Onsager-coefficients and form of the correlation-functions remains same as
those obtained by the relativistic Navier-Stokes equation and it does not
depend on any specific model of hydrodynamics. Further we numerically
investigate evolution of the correlation function using the one dimensional
boost-invariant (Bjorken) flow. We compare the correlation functions obtained
using the causal hydrodynamics with the correlation-function for the
relativistic Navier-Stokes equation. We find that the qualitative behavior of
the correlation-functions remain same for all the models of the causal
hydrodynamics.
