Some theoretical analyses and experimental investigations are performed to give a solution to the separation of tooth meshing, which is accompanied with alarming noise or occasionally abnormal noise. Tooth separation occurs at the specific speed defined by the amount of transmission error and tangential load. The more transmission errors are or the less the supplied load is, the lower the speed, at which separation occurs, may become. The largest dynamic load and the highest sound intensity level are presented immediately after the separation occurs. Noise spectra are elucidated in comparison with tooth behaviours. It is considered that an unusual subharmonic noise would be caused by abnormal meshing of every second tooth or every third tooth.