Induction motors run at (synchronous – slip) speeds determined by load. They are usually specified with about 5% slip. Unbalance vibration from the motor can be expected in a band +/–5% of the motor design speed. That will be 0.95 * supply_frequency / poles, where the number of poles is usually 2 or 4 but can also be 6 or 8.
While the vibration from motors is at motor RPM frequency, where the driven load is coupled through V-belts, load imbalance vibration will be centred on frequencies determined by the ratio of pulley diameters.
Magnetostriction in transformers will generate a buzz at twice the fundamental frequency. For 50Hz supplies that will be 100Hz, while for 60Hz supplies it will be 120Hz.