We're using BIAS SoundSoap Pro here, but most of the professional audio restoration software packages allow you to do the very same thing. A good technique for confirming your specific noise reduction settings is to switch SoundSoap Pro into the "Noise Only" listening mode. In this mode, you will hear only the noise that is being removed using the current noise reduction settings. If you're not hearing anything, then your settings are wrong. If you hear too much of the audio you want to keep, you can make adjustments from within this mode to back off on the processing. By toggling this feature on and off, you can effectively find the best settings so that you eliminate most of the noise without adversely affecting the audio you are trying to "clean up." Once you feel you have the best possible balance, go ahead and apply the settings, but save the file with a different name so that you won't lose the original. This is important because if you process the audio and then save it, you will most likely be writing over the original file. This would make it impossible to start over if the results are not what you expect. Typically, you can undo the processing, but only up until the point that you save the file, which then writes over the existing audio.