I bought an ESS Audio drive ES1868F that looked a little dirty in the photos, which is pretty common for parts of this vintage. After washing it on the sink however, I discovered that it wasn't dirt but some very thick corrosion. The damage is isolated to the back of the board, and only on a few parts; mostly passive, so I expected that it would still work.

After a bit of driver searching and setup, I can confirm that it does indeed still work. But there is a definite hum from it to my speakers when the volume is raised to a moderate level, and it is clearly audible when the volume is turned up to the maximum. I found that the hum is coming from the ground plane, as the hum occurs just the same if I simply touch the speaker plug to the PCI bracket instead of plugging it into the socket.

I was able to work around this issue by using a ground loop isolator between the card and the speakers. It wasn't just diminished either; it was completely gone.

My question is whether this hum is a fault in the design of the board itself or whether this is related to the corrosion. I'm thinking about trying to scrape some of it off, but I'm not sure if I want to replace any of the parts if it's working as it is, unless there is something I can do to I improve on the hum, or find a way to remove the need for a ground loop isolator.

Here are some photos of the card and the damage.