Vibrations are caused by out-of-balance forces. The source of the out-of-balance condition may be unequal rotor mass distribution, inadequate blade tracking, and or unbalanced drive train components.
Low frequency vibrations are generally related to the main rotor and are often due to an out-of-track bade. This results in a one-per-rev vertical vibration. Medium frequency vibrations are generally related to the engine and transmission. High-frequency vibrations are related to components that rotate at high RPMs. For most helicopters, this relates to tail rotor and its components. Vibrations originating from the tail rotor are usually felt through the pedals. Any abnormal vibration should be investigated, as the vibrations can cause significant damage to the aircraft.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-21
Principles of Helicopter Flight, 2nd Edition, pg. 181