Thank you @giuliomoro
It's probably about the ground loop due to the ground going both via USB and audio.
Yes this seems to be the case. I hear the noise when my Bela's audio out is plugged into the audio interface (Behringer UMC22). But if I plug headphones into the bela's audio output, it's a clean signal. Any suggestion how to avoid this ground loop? Does it have to do with the input impedance of the audio input jack on the audio IO? Unfortunately the audio interface only can be powered by USB power (no AC adapter input) ...
this should only happen if you are also connecting your mac or soundcard to Bela via an audio cable.
Yes I have both my Bela and audio interface plugged into the computer via usb. And the bela is sending audio to the audio interface.
no this won't work. You could use ethernet or a wifi dongle to access the IDE. Do you need USB for MIDI Connection?
I actually don't need to access the IDE. I need to send data from the Bela's 8 analog inputs to PD running on my Mac. (I can't run this PD patch on Bela because of processor limitations).
Overall, it seems that if I can avoid this ground loop, the problem will be solved. Any suggestions?