Where does that modulation comes from?

[FabienG]

Hello,
I am starting with the CS-80, but I went through the manual, tried different things, and I can't find out what's happening.
With the preset C.P_Mod_bass1, there is a slow modulation on what seems to be the filter. I tried to desactivate modulations in the "modulation select" panel, and change things in the VCF envelop, but there is still frequency jumps.
Can someone explain to me?
Thanks a lot.

[jeffbart]

That patch has sub oscillator mod using a random value, as well as PWM. It also has some detune between the 2 channels. All those contribute to the modulation you may hear.