Vortex Sound Research

I have a Korg Volca Keys and I'm noticing this very low-frequency modulation when I use the unison settings. I can replicate it by turning down all the knobs (except the sustain and cutoff at mid, just so you can actually hear something) and holding any key. What am I hearing from the oscillators here, and why does it happen? I recognize this kind of modulation from an electric organ I have at home, so now I'm pretty interested.