Hmmm, pulse width? The slightly detuned stacked square waves? Honestly I cheated that a bit, they're literally just two stacked sounds taking up two channels. The main one is at full volume while the detuned one is at about 25%. Then to add to the effect I gave both channels some vibrato at different depths. It's not the most efficient use of space, but it works!

Thanks! I think I will, actually. I still have some more ideas for it.