Off-stoichiometric Ni2MnGa polycrystalline films, deposited by the flash-evaporation technique on the heated mica substrates, show a martensitic phase transformation at 310 K. At room temperature, the films have a tetragonal structure (a=b=0.598 nm, c=0.576 nm) close to the bulk Ni2MnGa with c/a=0.96. The austenite to martensite transformation brings about an anomalous minimum in the effective magnetization and a strong increase in the ferromagnetic resonance linewidth in the martensitic phase just below the transformation temperature. The results are discussed in terms of a qualitative model that combines the ferromagnetic resonance response with a specific microstructure of the polycrystalline Ni2MnGa film.