I was fiddling with a microphone stand and this weird sound started coming out of the monitors.
I looked around to see what was making it, and it was my bare toes touching the plug on a a guitar cable.
The signal path:
guitar cable->Behringer Vamp-2 (Set to high gain, auto-wah and reverb)->Computer->Mixer->Monitors
I recorded this with my zoom H4. The plug on the guitar table was resting across my toes and the variation in the sound happened when I flexed my toes slightly.