Joakim wrote on 2021-11-18, 16:45:
I don't actually think it's interference. Rather some failing component.. but I'm no expert. I tried running the computer outside it's shell with battery or without..also tried some makeshift shielding methods between the motherboard and the sound card.. maybe need to replace some caps?
Look, it's just a guess, but the only thing that could "catch" a magnetic interference would be a coil. If a coil has lost its protective enamel, it may be picking up some noise, and changing its magnetic field, this could make a sound card make noises, it happens a lot with desktop motherboards with low build quality, it is possible to hear a noise that looks like a C64 loading a program.
Now, if the sound works perfectly on the batt, and plugged in doesn't work, then maybe you have a problem with the internal PSU. there is a sector that is responsible for "switching" the power of the PSU and the battery power, it is usually found on the motherboard and not on the PSU, it must be close to the main battery connector.
edit: what do you get if you use headphones?