Are you using an internal sound card? It might be electronic interference between the GPU and the sound card, and so you're actually listening to what the GPU sounds like when it's running its part of OBS.
Another possibility is that you have a ground loop somewhere in your rig, that is picking up a switching power supply somewhere. At that specific load, the switching frequency of that supply gets into the audible range.
I've had that happen with a laptop and a professional audio rig, even though I was using a USB connection between the laptop and the (otherwise analog) sound board. "USB is digital, and digital is immune to that, right?" Erm, yes and no. As soon as the USB signal got converted to analog so that the rest of the analog board could use it, the ground loop noise got into the analog version. I knew it was a ground loop when I unplugged the laptop from the wall (running on battery) and it went away. Fixed it with a USB isolator. NEVER BREAK A SAFETY GROUND!!!
