Hi All,
The point is that returning to the mantra is not a hanging on to any particular experience of it, like a vibration, a repetition, with or without spaces, etc. What we are returning to is the process of refinement of the mantra to less and less. Hanging on to any type of experience is not what it is about. We pick up the mantra wherever it is in the mind and let it refine from there. We can’t let it refine if we are hanging on to an experience. Gotta let it go the way it will. It is a very natural process. Anything we try to add to it will interfere.
And if the mantra is just staying where it is, not seeming to refine, that is ok. Sooner or later it will disappear, eventually to be noticed as gone, and then we go back to the mantra wherever it is in the mind. That is correct meditation.
All the best!
The guru is in you. 
Hi qspadone and all,
A continuous sound vibration is a sound you can hear that does not have a beginning, or end. Like the hum of a bumble bee.
Continuously imposing back-to-back repetition means using your will to force the repetition of the mantra to happen with no pauses in between each repetition. When we practice, we should start the mantra of back-to-back and then let it go as it will. So, the let it go as it will part means not imposing gaps between mantra repetitions, and not imposing back-to-back repetition. Basically, there is no manipulation by the mind, but instead we simply allow what happens to happen. Then if you realise you are off the mantra, pick it up again back-to-back and let it go as it will.