Check the Circuitbenders FM Mod here:
http://www.circuitbenders.co.uk/synthmod/POLYEX.html"FM MOD'S:
These consist of a source selection knob and an FM amount knob. Essentially VCF FM allows you to modulate the filter cutoff at audio frequencies to create all kinds of clanging cross modulation and bizarre alien splatters & sweeps. This effect works best with the resonance cranked up into self oscillation which is where the new resonance knob mod comes into its own. The source selection knob allows you to select one of four sources for the FM signal. These are DCO1, DCO2, Noise and audio input. The audio input takes its signal from before the input level knob so you can use the audio to modulate the filter cutoff but remove the actual audio signal from the mix. The source knob also has an off setting."
That's interesting.