Rdlagrand,
“With complete silence? or a feint hum?”
Whether in complete silence, or a quiet hum, whether it is a mantra or any other object - that is not the essential phenomenon. What is essential is to keep the mind focused like a lens on a single point, remaining a witness without interruption, without any gaps of space. If the mind becomes distracted, you gently bring it back again onto your object of concentration. If the mind becomes distracted yet again, you yet again bring it back onto your object of concentration. This bringing back of the mind again and again onto the object sharpens your jokiri, the concentrative power of the mind. Once your awareness is flowing into the mantra with one-pointedness over a certain amount of time, then the mantra disappears from your vision, and one’s concentration becomes transformed into meditation. Then one will simply be aware without focusing the mind on anything in particular, but aware in such a way that you are not even aware that you are in meditation - there is simply the awareness itself. In fact the moment you even reflect that you are in meditation, in a split moment you will come out of it.
It may be useful to repeat the mantra silently, but as fast as possible in the mind at a speed which cannot be done when when you are repeating it verbally. This is to allow the mind to remain focused on just a single object without any other thought or perception interfering.