Frequency of motor vibration with different voltage
Being a DC motor, it's probably proportional to the voltage, more or less. So the quoted 11,000 RPM at 5V is around 180Hz, I would expect 100Hz at about 2.8V. 

If the voltage is too low, the motor will stall and you'll get 0Hz. 

Edit: Here's an excerpt from a datasheet for a similar (lower voltage and 12,500 RPM @3V) device: 



As you can see, it varies from 220 Hz at 3V down to about 100Hz where it stalls.