The Hummingbird is usually controlled by Twitter, I fancied playing it myself so rigged up a simple switch mode using connector block and bike brake cable that runs to the other pin of each relay unit.
The noises are a result of electromagnetic interference from the motors creating a current in the pickup in the middle. The pickup is actually a transformer coil but as it uses the same principle of physics as a guitar pickup, it works just fine for noise creation. The different motors create a different tone of noise and when combined the tones mingle and mash and create slightly different tones. Also, as they all run from the same power source, as another motor is added to the circuit, it drains the other motors of current which slows them down slightly and alters their pitch.