At least it's just a problem in the Eurorack cab itself. This reminds me of a much more confusing "noise" issue I ran into in the studio back in the mid-80s...
I was setting up some mics on a piano. I'd put up a stereo pair of AKG 414s overhead for the overall sound, a couple of AKG 451s aimed inside the piano, and I put a Shure SM81 underneath to capture more of the cabinet resonance. Walked back into the control room, ran up the channels to check for room tone issues...and waaaaaay down low, I could hear a...piano?
OK, there wasn't anyone playing it at this point. Piano's still there, though...so I pushed the input pre and fader to the max to see what in the hell this could be. And I heard the same four chords being repeated...and a voice saying "Radio Canada International" in English and French. And I knew EXACTLY what that was. It was only on that Shure, though...
I went back out, swapped the SM81 for a different pencil condenser, went back into the control room...no piano noise.
So what WAS that, anyway? Well...somehow, the SM81 was picking up a shortwave broadcast transmitted from Sackville, New Brunswick, Canada on Radio Canada. Now, if I lived right down the road from this, it might make sense. But this was out in the Nashville burbs, over 1000 miles distant. Suffice to say, I tagged the mic for the studio tech to deal with, and since then I've got a rule that I DO NOT use Shure condenser mics, period.
Moral of the story: noise chasing isn't the easiest thing, and often the solution is far from intuitive. Had I NOT had a long background of working with amateur radio, shortwave listening, and such, I probably wouldn't have figured out what was happening, as you'd never expect an RFI issue like that.