If you have any idea what it could be, I'd love to hear from you.
-- Hoggres
I don’t have a definitive solution, but I do have a few suggestions to consider:
Approach 1:
Unplug all modules from the power bus. Reconnect the modules one by one. This might help identify if a specific module is causing the noise. If you find the source, consider using a "Noise Killer Plug" on the problematic module: https://www.midi-hardware.com/?section=prod_info&product=NKP
Approach 2:
Take a loose wire and connect one end to the rail of your Eurorack and the other end to the casing of your other equipment (e.g., audio interface, amplifier) or any component involved in your audio signal chain.
Approach 3:
Start by simplifying your audio path to isolate the issue.
Begin with the simplest setup: Eurorack -> amplifier.
Gradually add components: Eurorack -> audio interface -> amplifier. Adapt this sequence based on your actual setup.
Approach 4:
Try using a different sound system. Take your rack to a friend’s place and test it there. If the noise is absent, there might be a device in your home setup (e.g., a refrigerator) causing interference. Rotating power plugs by 180 degrees might help (if your country uses reversible power plugs).
Personal Experience:
I have a "WMD Performance Mixer Mk1" that produced significant noise through my speakers. After connecting this module to a different power supply in my rack (I have eight Meanwell RT-65B units), the noise disappeared. Replacing the Meanwell RT-65B made no difference. This suggests that certain module combinations within the same power supply can create noise issues.
Good luck!