Digital (?) noise on output of Ambika with 3*SVF

Hi,
I have just finished 3rd SVF card for my Ambika, and I can hear nasty noise/scratches when playing 3-note chords. This is not audible when playing 1 or 2 notes.
Interestingly, this also happens when I set patch volume to 0. The voice is off, but I can still hear the noises, at the same level. That makes me think that the digital parts of the signal get to the analog path, somehow.
Still strange that it happens only with 3 notes, though.
Anyone experienced that? Where to look for the root cause?
This can be heard on the master out as well as when using individual outputs.