This is crazy, but I’m having the EXACT same problem with only ONE of my three Saturn consoles through the OSSC.
I’ve had both of my model 2 Saturn units hooked up for a while through my OSSC. No issues at all.
Today: I hooked up my US model 1 (to get a game save off it), and noticed the hissing sound. Just like Gummi said, it only shows up when the Saturn is playing a sound. I connected my Saturn audio *directly* to my receiver and the hiss went away. Connected through the Framemeister, my CRT, directly to a headphone amp… no hiss. As soon as I run it through the OSSC, I get the hissing sound.
I’ve used 3 different SCART cables and 2 different sets of HD Retrovision component cables. So I can confirm that it’s not a cable issue.
It’s the craziest thing.