A method of determining variations in operating characteristics of a mechanical system having a rotatory mode of operation. There are provided the steps of first monitoring operating impulses generated by the mechanical system during a first interval of operation; first analyzing the operating impulses obtained during the first interval to determine the intensity and frequency of the operating impulses; first correlating the operating impulses obtained during the first interval to corresponding angular positions of the rotatory mode of operation; and first producing a first record of the intensity and frequency of the operating impulses obtained during the first interval correlated to the corresponding angular positions of the mechanical system. These steps are repeated to form trend data that results from comparing the first and second records to determine differences in the operating impulses obtained during the respective first and second intervals correlated to the corresponding angular positions of the mechanical system.