The AC source is where a mechanical shaking machine attached to rigid building structure frame is purposely ran out of balance. Frequency and size of the vibrations can be adjusted. Above a meter or microphone able to sense small vibrations and their strength.
Annoying vibrations may be traced to neighbor's machinery and coming through the ground to another structure to give vibration resonance, unbalanced machinery within the building, certain wind gusts, subway or street traffic, even elevators in the building.
Also, if your shaking machine brings the building down you might feel like Samson.
