Audacity Leveler to reduce background noise

Leveler is the compressor-limiter function of the free and open-source audio editing software Audacity.

A secondary feature of Leveler called Noise Threshold is extremely useful in reducing background noise.  You can fool Leveler into subtracting most of your background noise by first using Amplify to set the volume of your audio to a new peak amplitude of about -20 dB.  Then set Leveler to Light, and the Noise Threshold to -20 dB.  When you click OK, Leveler automatically deletes the softest sounds, such as background hum.  Then you can use Amplify to raise the volume again – the sounds that have been deleted will not come back.