Phasing instrument and drift field · Free, runs in your browser
Two voices play the same sound. One of them runs slightly faster. That is the whole idea, and everything else follows from it. They start locked together, edge apart a millisecond at a time until what was one sound becomes two, turn into a rhythm that nobody wrote, and eventually find their way back to where they started.
The voices are hard panned left and right, so the gap opens across the stereo field as well as in time. People were doing this in the 1960s with two tape machines and two copies of the same loop, and it worked because no two machines ever run at exactly the same speed. The drift was a fault in the equipment. Somebody noticed the fault was more interesting than the loop.
Use the built in step sequencer, or load your own audio and both voices will loop it. Stereo files are folded to mono first and you choose how, with a correlation meter to tell you in advance whether summing will hollow your file out.
The Drift Field modulates the rate of drift itself, sweeping between two values over anything from five seconds to fifteen minutes. Set it to cross zero and the second voice falls behind, slows to a dead stop against the first, then overtakes.
Renders offline, faster than real time, up to twenty minutes as a 48kHz stereo WAV that drops straight into a session. Set a slow drift, render longer than you need, and go hunting through the file for the ninety seconds where it did something you would not have thought of.