I have only heard nada three or four times, and a very long time ago (when at the ashram). So far it is always in the "left ear." (As opposed to when Lord Krishna breathed his divine state into my right ear... ahhhhhhhh, yumm).
Until this time, the nada was divine, exquisite tinkling, like tiny bells, I could not describe how beautiful.
Almost simultaneously, I realized the hum reminded me of the pace and vibration of the tamboura, a familiar instrument played live after chanting (at the ashram) to facilitate slipping into the purified place prepared for meditation. And I realized that this hum was OM. This is the subtle vibration of OM. It is AAAAUUUUUMMMMM. Not a breath. Simply a continuous vibration at the center place.
Again I was amazed at how an Indian "musical" instrument is and was constructed to evoke a sound so divine, so subtle, so essential it cannot be described with words.
It took me to a place so sacred, I was awed and almost confounded. I didn't think of the journey to the heart as discovering, stumbling really onto a place that vibrated, where you could experience a singular vibration, without any other(s).
The sacred sound OM is not the sound of it, but rather the description, like a marked gate. I am still awed. It was the sound of living consciousness, of Paramashiva.