Audacity's built-in noise reduction effect can be used to attenuate background noise such as tape hiss, fans or hums. It will not work very well for removing loud noise like talking or music in the background. There are two steps:
- Show Audacity a noise profile - a short section of audio containing only the noise to be reduced
- Return to the effect to apply the noise profile to all the audio.
For more detailed information please consult Audacity's noise reduction webpage.