had a similar issue:
https://forum.bela.io/d/1524-pepper-questions
i found it crept into the rest of my system, possibly modulating the ground line.
which, in my understanding, may point towards ground loops, insufficient power sourcing, and insufficient power sinking (ground lines) which tends to get sorely overlooked.
eurorack power distribution is by no means an ideal standard - and mixing digital and analog modules is an added risk, possibly, if it's at all comparable to my experience with guitar pedals, where i've had digital clocks leaking into analog units over daisychains. which is essentially what eurorack power buses are.
what fixes most problems in the guitar world is isolated power supplies per pedal.
not quite an option with modular.
i've been meaning to run some tests with heavier supplies, but also heavier gauge ground wiring and star grounding schemes, to see if i can improve matters. not just for Pepper, but making my rack futureproof, as more and more digital modules are hitting the market..
just to say that i'm leaning towards blaming the power distribution more than anything else.