If you don't know why, then why worry about how ;-)
just kidding.
I think that maybe you are talking about the fact the motors will sometimes give a vibration which beats.
That can either be due to interaction of a harmonic of running speed with the 2*LF vibration, or else it can be due to pole-pass modulation which occurs when rotor bars are broken or when there is dynamic eccentricity. See a recent post entitled RBF.
I'm not sure if that's what you're after. If you have specific additional questions please ask.
