Recently I have been having a problem with the noise reduction feature.
Whenever I try to get rid of background noise from my audio, it sounds like it is coming from a muffled low-quality microphone. However, if I amplify the audio without noise reduction, the audio sounds very clear (aside from some background buzzing).
That can happen with noise reduction. You can experiment with the settings, but noise reduction works best when you have a constant low-level background noise… It works best when you don’t really need it. There’s a reason pros still record in soundproof studios with good equipment.
If the buzz is related to the power line frequency you can try notch filters at the line frequency (50 or 60 Hz) and at the harmonics (120Hz, 180Hz, etc.)
I would venture to guess your problem has nothing to do with Audacity. It’s the preamp of your microphone. Weak preamps add that white noise hiss, and the higher you raise the gain on your microphone, the higher you raise the noise.
Plug your microphone into a good preamp like a Cloud Lifter or any name brand channel strip and this noise will vanish.