The A320 has had problems in the past with Limit Cycle Oscillation of control surfaces. Perhaps the A330 does too.
Essentially slop can build up in actuator attachments due to wear and a control surface can limit cycle oscillate over a small extent extent due to accumulated slop.
If attaching points are still in wear limits, maintenance can then rig in some aerodynamic bias which will hold the surface hard against the actuator.
An inspection of all control surfaces that are visually accessible inflight might be enlightening, but careful execution of the vibration event form is essential.
It would be helpful to hear from an Airbus maintenance specialist regarding that part of the problem solution.