I was having horrible sound quality on my PC and spent hours on forums searching for possible resolutions. I checked several software solutions and became convinced that it was a motherboard or electrical issue. I tried a different outlet across the room, but heard the same noise. I tested the wiring of the outlets and they seem to be wired correctly.
The noise started very suddenly after having been quiet. Even during the boot process, before any drivers can load, there is an odd hissing back and forth in the headphones with odd bloops and beeps now and then. This gets worse after the OS loads (Linux or Windows). Even when sound is muted on the OS, I still hear the hissing and blooping. I did test the headphones on a different device (battery powered) and they work fine. I also tried a much older PC on the same outlet and it only had a very faint rustling sound in the background, leading me to think it is a motherboard issue. Even a ground-loop isolator doesn’t change anything, so it doesn’t seem to be a grounding issue.
The PC and associated equipment are connected to a UPS (battery backup) which also helps condition the power.
It did turn out to be the motherboard having a problem. I bought an inexpensive PCI-e audio card (ASUS Xonar DSX) and the sound is perfectly clear now. It took a bit of looking at settings to make it work in Linux, but it was recognized right away and was mostly a matter of adjusting settings in alsamixer and pulse-audio. The PC motherboard is a 10yr-old “ASUS P6X58D Premium”. Still works fine for what I need, but probably time to look at building a new box. I disabled audio in BIOS so that nothing would interfere with the add-in card.