A normal spring (for example in my ballpoint) is made of particles that interact with each other wich makes the string vibrate with a frequency that depends on the material (the particles).
Now a string in string theory doesn´t consist out of particles between wich a force is working. No parts of the string exchanges a force carrier particle with other parts of the string, like particles that constitute a normal string do with other particles of the string in, say again, my ballpoint.
Then how can a string vibrate if there aren´t forces at work in the string? Or is this vibration just an analogy taken from real springs, without explaining, as can be done in real springs, why the string can vibrate? Can string theory account for the vibrational motions of it´s elementary entities whose different vibrational modes can explain the different elementary particles and their charges and thus the interactions of elementary particles? I can recall reading two of Brian Greens books; in one book he writes that the masses of elementary particles are due to vibrational modes, and in another book he writes it´s the Higgs-mechanism that does the job, but this aside.
So the main question is simple: what makes a string vibrate?