A drive system of a gas turbine engine includes at least one drive shaft operable to rotate within the gas turbine engine, a phonic wheel coupled to the at least one drive shaft, a speed sensor operable to detect rotation of the phonic wheel indicative of rotation of the at least one drive shaft, and a processing system coupled to the speed sensor. The processing system is operable to detect a phonic wheel pulse train indicative of rotation of the at least one drive shaft via the speed sensor, determine a torsional mode of the at least one drive shaft based on the phonic wheel pulse train, and record one or more trends of the torsional mode indicative of a health status of the drive system.