Oh my bad. I just have the soundcard plugged in and recording a mono channel (channel 1) from the soundcard. There is nothing attached to the soundcard except it's USB wire being plugged into the PC machine so I'm recording nothingness. I'm doing it to see if the hum was coming from the mic, or preamp to rule those out. I tried both channels, and the hum persists.
Basically debugging to see what the cause could be.