I was wondering does that even make any sense. Basically, driving a VCF like a VCO in an FM synthesis chain. Theoretically a VCF can produce a sine tone based on the resonance, but also more harmonic. I guess people do it all the time with envelopes and LFOs, but is there any reason that it can’t be driven at higher frequencies? I don’t see many synths that support that out of the box. I know some filters are slow, so maybe that is the reason, is that the case with digital filters though? Are there any vcv rack VCF modules that would be better for testing this than others? I feel like it would help tame some of the overtones in complex FM chains.
The VCV filter module will certainly oscillate at audio range, if that’s the question. Turn the resonance and drive all the way up. Whether you can do audio-rate frequency modulation through its V/8ve input I don’t know but I will in about a minute
Yup, you sure can
If I feedback into several chained filters it sounds like a herd of bison.
Exactly!
The Korg Opsix has a Filter FM mode alongside traditional phase modulation, ring modulation, and wavefolding. I don’t have first hand experience with it, but it’s worth a look if only for some patching ideas.
every synthesizer in the 70’s could do this, too.
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I wish I could walk into Best Buy and play with synths from the 70s! 
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here is a patch i did partially based around this idea: black mangrove v2 - YouTube in this case the idea was to use sequential switch to alternate between fixed reliable parameters for slightly weird fm bass and freely messing around with the sound via midi sliders for weird squelch noises.