electronic musician has put together an overview of the most common forms of noise and other forms of damage that can occur within a recording. The article, as well as describing the form of damage, also explains some of the theory behind the methods used to remove/fix it.
This is probably a useful read for anyone who has some less than perfect recordings but is unable to re-record and so needs to 'clean up' their recordings.
Following on from the above article EM has a related article here which looks at how to use audio restoration software correctly to get the best results.
Broadband Noise - Spoken word & Music
Pops, Clicks & Crackles
The article goes into pretty reasonable depth and contains advice from 'professionals'.
Some of the key points:
* Always preview before applying destructive processes. Make sure you have a backup copy of the audio.
* If your software allows it, pop the AR processing in and out to compare it with the unprocessed audio.
* Don't rely strictly on automatic settings, especially in broadband-noise reducers. Experiment with the threshold and reduction sliders (or equivalents) as well as parameters like attack and release. If you start hearing artifacts, back off on the amount of the effect.
* Try to find as long a sample of noise for the “learn” function as you can. It will help with the accuracy of the settings.
* Use the “noise-only” monitoring feature to check the part of the audio being removed by the software.
* In heavy noise situations, consider a multiband expander or an EQ boost set to the frequency range of the target audio to help bring it out.
* If the audio contains rhythmic material, make sure the AR software isn't degrading the transient response.
* If you don't have declipping software, try de-essing in the area of 2 kHz (and higher) to minimize the distortion artifacts of clipped audio on spoken-word tracks.
* When editing out an anomaly from a file being used in a soundtrack situation, don't delete audio, or you'll change the timing. Eliminate it by reducing level instead.
* Don't edit out time when cleaning up a video soundtrack, or you'll likely affect the sync. Use attenuation to get rid of glitches instead.