Sorry. The AN1X gets a little confusing because in the Mix/VCF sectiion there's a dedicated high pass filter, plus you can also pick a high pass filter from VCF type.
Go to the Mix/VCF section, go to VCF type, choose high pass filter. Then go to the VCF section, turn the filter way down, turn the resonance way up (and the volume down) the sweep the filter with a saw tooth lfo, slowly. It sounds like the filter is self oscillating.
Jimmy