Helispotter,
I mean it is not related to any rotor harmonic or sub-harmonic, so no .5, 1, 2 etc /revs. These are generally at a natural frequency of the rotor, pylon, and/or airframe. For an asynchronous to become a problem the “and” is the operable term. It generally takes a gust exciting the rotor at a frequency close to a fixed system mode which results in a response in such a manner that it can feed back into the rotor to sustain or amplify the vibration.