How to trouble shoot or fix the sensor cleaning mechanism of T3i?

My camera no longer makes the sensor cleaning noises when I turn it on or off (automatic cleaning is enabled in the menu options). If I turn it off/on in a silent room I can sometimes hear a very feint click. Is there a step by step trouble shooting guide for the cleaning mechanism or a simple fix (if this is a common issue)? I'm hesitant to start poking around in there without a reasonable idea of what I should be looking for but from what I could see with the mirror up, there is no obvious damage and the camera works just fine otherwise.

I made the terrible mistake of loaning someone my camera and they in turn lent it to someone else and neither admits to any mishandling so I cant trace the issue to a specific incident.