Audacity has a noise removal feature that lets you select a few seconds of what you consider noise, and identify that to the program as noise. A good spot for me is areas in a recording that are supposed to be silent but generally aren't if you're recording mic input live. Then you select the whole track and run the noise removal on it. Works well enough for me.

As I mentioned above, unless they have made sudden huge leaps in quality of their noise reduction process it isn't really a good quality tool. I know from conversations with Ricardus some of his uses and they are similar to what I have done in the past as well, for example restoring old analog recordings, etc. Where depending on the source material you have to have a pretty dang minimal artifact experience(Recordings of high dynamic range classical music for example), and also depending on the source material the exact needs may change over time, requiring automation to use effectively. Neither of these applied in my experience with Audacity's tool.

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