Could be picking up crosstalk from something else. That seems to be a common thing with these kinds of noise circuits; I had a kind of similar problem recently: Noise source circuits - #7 by analogoutput complete with the now-you-hear-it, now-you-don’t. No guarantees but if the problem recurs you might consider filtering the +12 V rail by replacing R30 with two 470 kΩ in series and then adding a 1 µF capacitor from their junction to ground. I don’t think that would do any harm (certainly nothing permanent, just unsolder the cap if you don’t like it) and might be a fix?
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