Charged-particle products with ~3.9-MeV energy were observed in a low-energy experiment (Ep 330 keV) with a proton bombarding a Ti2Hx target. The features of the charged-particle products were the same as those of an alpha particle. The threshold of the reaction was ~150 keV. The maximum reaction rate reached more than 105 r/s, while the proton energy and current were 324 keV and 1.2 mA, respectively. The excitation curve of this unknown reaction increased exponentially with the growth of proton energy. There is no known nuclear reaction induced by a proton that can be applied to interpret this experimental phenomenon. Some interpretations, e.g., an indirect secondary reaction and a multibody reaction model, are discussed, but the origin of this unknown nuclear reaction is still a mystery and under study.