  We develop a framework of causal hydrodynamic fluctuations in one-dimensional
expanding system performing linearisation of the hydrodynamic equations around
the boost invariant solution. Through the description of space-time evolution
of thermodynamic variables and flow velocity, we find a novel phenomenon that
the structure of thermodynamic variables is almost frozen. We also show that
two-particle correlation functions of final hadrons after freezeout are closely
related with the mass of hadrons and properties of the medium such as
viscosity, relaxation time and equation of state.
