Open your audio file in Audacity.
Highlight the initial portion of your audio file that contains the unwanted tape hiss, fan noise, hum, or other kind of noise that you want to eliminate. You will have to zoom in considerably on the sound wave to make your edit exact.
Open the "Effects" menu, select "Noise Removal," and click "Get Profile." Audacity will then learn the kind of sound that you want removed.
Select the rest of the audio file and click the "Remove Noise" button. This will clean up the rest of your audio file.