I suspect that there is a ground track that is carrying grounds from several places; the very tiniest of ground current will create hum or buzz. The stage that is being affected must be the compression stage and it is clearly only on one side.
I’m sure that if I had the unit here I could isolate the problem and re-route the ground, but the advice is all I can achieve at the moment.
This could be an example of why circuit board layout is so critical in audio circuits; there are so many variables that it’s very much an art, and we don’t get it right all the time! But it’s interesting that I have not come across this before…. and I have looked closely at the compressors I have here and such an effect is not apparent.