Hi Split
in my world, whirl is the most common cause of subsynch vibration although pulsations from other sources is right up there too.
Karassik (Pump Handbook) has a good table showing the relative frequency of various vibration causes.
Other possibilities include: A rub, flow induced instability, check bearing cage fault frequency. Also,as the pump is loaded, additional stiffening occurs from internal seals and wear rings. This could increase the resonance frequency.
