I recently shot some video footage and I used the camera's mic (I know it was a bad idea, but...). Unfortunately, there was some on-going background noise (I think it was a fan). How can I get rid of the noise?
I use the Audacity Noise Removal tool for this all the time. It's a little tricky to figure out, but it does a great job once you know how to use it.
It's a two step process - first select a section of the audio that has nothing but noise. Then in the Audacity Effect menu, select Noise Removal, then click Get Noise Profile.
At this point, nothing will have changed in your audio, you'll just have taught Audacity what's considered noise for the current track.
Next, select the entire track and select Effect / Repeat Noise Effect (on Windows, you can just press Control-R). This will apply your noise settings to the entire track. If you prefer to dial in the settings, you can instead select Effect / Noise Removal, tweak setting, and click OK, but I never bother with this.
In your audio editor, switch to spectrum view and try to identify the frequency band of the fan noise. Next you can either use a parametric EQ or a filter.
If you have a parametric EQ, you can easily roll off the top or bottom frequencies, or cut the frequency band that the fan occupies.
If you are using a filter, then use a low pass filter with a high frequency setting to cut out high frequency fan sounds. Use a high pass filter with a low frequency setting to cut out low frequency fan sounds. Use a band reject filter to remove a specific frequency range in the middle.
If you don't already have an audio editor, get Audacity. Be sure to get the audio plugin packs as well.
I'm having some trouble with my copy of Audacity, I'll update this when I've reinstalled it with the latest plugins.