Multiple layers of MoS2 are used as channel materials in a type of field-effect transistor (FET). It was found that the hysteresis in transfer curves and low-frequency noise (LFN) characteristics are varied by the number of layers in MoS2 due to the different influences of bulk traps. The LFN characteristics of a FET composed of a 'bi-layer' MoS2 channel, which was passivated with an atomic-layer-deposited (ALD) Al2O3 layer, follow the conventional carrier number fluctuation (CNF) model. However, FETs consisting of multi-layered MoS2 channels (4, 7, 9, and 18 layers) show abnormal LFN characteristics, which substantially deviate from well-established 1/f noise models such as the CNF and Hooge's mobility fluctuation models. The bulk traps inside the MoS2 layers are the origin of the abnormal LFN characteristics and the large hysteresis of FETs with multi-layered MoS2 is due to its randomly embedded bulk traps. Secondary ion mass spectrometry (SIMS) confirms the existence of oxygen species that induce the electrical bulk trap in the MoS2 layers.