I had just finished with my daily NAP working with Elubatel and Nitika. I settled down in mediation afterwards had images flowing in and out. I was going pretty deep when there was a what sounded like a a pop you might get if you had crushed a plastic bottle and it had popped as it expanded back. That's the only way I can describe it.
I couldn't find a physical reason it happened. So there you go. My first spirit sound during a NAP ritual.