Hmm, interesting. There's no explicit source of sound, here, yet there's lots of sound to be heard. I take it there's just enough noise at the startup to get things started and feedback loops amplify it and off she goes. I wonder if this initial sound is a "mistake" in AM, some artifact that shouldn't be there.

You are correct to observe that there is something going on, but there is no "mistake". As with analog systems, AudioMulch's feedback loops are not entirely noise-free. For technical reasons (relating to something called denormal underflow) AudioMulch injects a very very small amount of noise in to feedback loops. When a feedback loop's loop-gain is below unity this noise is undetectable. However for high gain loops (such as in this patch, and some other feedback patches) the noise can cause build up. You can see that build up in this patch when it first starts.

You can get some nice sounds riding MCompressor_1's input gain in this patch to control the resonance, or flipping it from 0 to full to get new resonance attacks.

An effect I like to use in a FB network is the shaper and just bring up some of the harmonics or automate them.