A Hamiltonian system with few particles and a heavy adiabatic piston is studied. With ensemble averages based on proper classification of slow and fast variables, a nonequilibrium state is defined and the relaxation from nonequilibrium to equilibrium is investigated. Coherent oscillation, dissipative oscillation damping, and anti-intuition energy transfer of the adiabatic piston are observed by numerical simulations. Noise-driven thermodynamic equations of slow variables are derived, based on the assumption of local equilibrium and the fluctuation-dissipation theorem, to understand and quantitatively reproduce all the above few-body nonequilibrium relaxation features.