[jrickert]

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.

[mgkimsal]

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).

[Epholys]

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.