I have experienced something I refer to as "Internal Vibrations". In my situation they occur randomly and are caused by anxiety, for which I take an anxyolitic (Venlafaxine). They are strictly "Internal" and do not cause any physical tremors. I do, however, experience true physical tremors, as well, from time to time, for which a Benzodiazepine (Clonazepam in my case) resolves them.