Dr. Quack and Mr. Chirp

[rockola]

I would have entered this in the Creative Audio contest, if I'd had the foresight to grab a longer video. It's a Dr. Quack on vero behaving quite strangely, making chirping noises, and not passing any audio through. I had two solder bridges in there, removed those and the problem went away. Unfortunately I didn't make notes and don't remember where the solder bridges were, which brings us to my question:

How would I go about designing a circuit that does these chirping noises on purpose? Preferably using a similar set of parts as Dr. Quack (i.e. a few transistors and an opamp or two, not looking for a video game effect IC or a digital sample playing solution here).