I had a similar personal experience. For years growing up I slept to a white noise machine that played a 3-second recording of a mountain stream. I became intimately familiar with the clip and could anticipate every trickle, the changing pitches of different water droplets, and the length of the loop. I can still conjure it from memory in great detail.

Intriguingly, on a single night I experienced some sort of hypnogogic auditory hallucination that caused the sound to lose all familiarity. Instead, I perceived the sound of a male singer arpeggiating rapidly. I found it highly disturbing.