I'm seeing some weird low-frequency oscillation coming from the Double Verb Vibrato AMP model.
The OUT1 block has a CS for the output level that seems to be interacting with the AMP block. Removing the CS or bypassing either block stops it. I use this same OUT block in a bunch of other presets without problem so it seems to be tied to the Double Verb Vibrato. I have another preset with the same AMP and OUT1 block that does the same thing. I suspect the sound is tied to the strobing of the FC6; Switch 2 has a hold function to toggle CS1.
The attached archive has a minimal preset, the system+gb+fc backup and the all-layouts controller export.
This could be tied to
https://forum.fractalaudio.com/threads/fm3-firmware-version-5-00-beta-1.180079/post-2201830.