Sound - Question and answers

I'm working on a home podcast setup with a USB condenser microphone. After recording, I import the files into Audacity and consistently hear a low-frequency hum, especially during quiet parts. I've tried using the Noise Reduction tool with a sample of the hum, but it only reduces it slightly and sometimes makes the voice sound muffled. I've checked that all cables are secure and minimized background noise in the room, but the issue persists. I'm looking for practical steps or software settings to resolve this without expensive hardware upgrades.