I have had similar experience to Ghop, with random thoughts, which often form into a kind of story (the kind of thing you get just as you're falling off to sleep). After sometime (don't know how long) I come out of this kind of dream sequence- sometimes with a bit of a jolt.
The syllable is usually more obvious at the start of the meditation, and most of the time goes along with my heart beat (sometimes with the breath). Often I feel the heart beat in my head along with the syllable.
Is this what you would expect during NSR meditation?