Some of the same stuff I used in "Zalgo's Paradox," combined with modular-synth ideas (though this is entirely digital, made with Csound) and made into a more or less complete track.
Cascading, overlapping tones; all 12 seconds long but the time spacing is a bit randomized, with the overlap slowly increasing over the length of the piece. Each is 249 cents below the last, with a random chance of jumping up an octave getting stronger as the tones get lower, so they always stay roughly in the same range. These are sawtooth waves shaped by flipping alternate bits in the digital value; that has the effect of boosting harmonics at the octaves, so it's a little bit like the Shepard tones from Zalgo's. Then it goes through a slightly resonant lowpass filter (just like the analog folks use - and that chops off most of the harmonics), gets a bit of filtered white noise added, and then the fun begins.
The concept here is that I've got automatically-spawning "notes" that do not actually make any noise themselves, but consist of adjusting the phase and frequency response on the stuff going on in the background. It's like painting a picture with Vaseline on a camera lens. The mixture of Shepard tones and noise goes in parallel into two allpass filters (flat frequency response, complicated phase response) and a delay line (which can be thought of as a third allpass filter) and the resulting three signals go into a couple of polarizing mixers, one each for the left and right channels. Depending on the settings of the mixers, different chunks of the spectrum get notched out. At 45 seconds in I play a "note" on a Csound instrument that modulates the mixer settings for a little while and then spawns a similar "note" for the right side a random distance into the future. Each modulation "note" unconditionally (until the end) triggers another on the opposite channel; but it also has a chance of triggering another on the *same* channel, as well as in a third instrument that swirls the mix around between the two channels. So the rate of modulation "notes" increases exponentially over time and would blow up completely if I allowed it to.