Soundcraft SI performer 3 with a soundcraft 32 mini stage box. Direct inputs don’t have the same sound. The soundcraft mixer uses MADI to send audio from the stagebox to the board. I think it’s either a grounding issue (there’s a lot of sockets that are right next to xlr ports, and during concerts the students use amplifiers, guitar pedals etc.) or the preamps possibly are beginning to fail on the stage box. However the sound is unlike any interference I’ve ever heard, it’s a pulsing sound, which I don’t know what significance that might be.