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.
never knew how to phrase it, but I've had that a couple of times in the past few years. I've had to get really picky abouth the white noise sounds I listen to. Eventually, almost all the loops become identifiable - I can tell when the loop restarts, etc. I had two different loops (both of water sounds - waterfalls, maybe? - iirc) that, at some point, started to register as human voices, and it freaked me out more than a bit.
fwiw, the only thing that works well for me is masking any other sounds with a mix of grey and brown noise on top. i use 'white noise app' to achieve this, and it helps me get to sleep, masking out many other ambient disturbances (road/car noises, mostly).
For everything about noises, I highly recommend mynoise.net. It has a ton of high quality sound generator which does not loop. If it is too monotonous, you can automatically "animate" the sounds which modify continuously the sounds at different frequencies.
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.