I am also having this exact same issue. I initially thought it was a possible power supply noise however i recently got a secondary travel case with a different output module and the issue still occurs. I believe the hum I am getting is between 5k and 10k Hz and it seems to get worse when the kickain is triggered and fades slightly witht the decay. I have also tried with no input from my mixer module with just the kickain going directly into my ouyput module and the hum still occurs