Audacity is a free open source audio editor. In order to import audio from a video clip you need to install the Audacity program and an additional file called FFMpeg. Here is a YouTube video on how to do this. The author also mentions using the noise reduction tool in Audacity. Here are the steps I follow after importing the audio: 1) select a small section of the audio file that only contains the noise you want removed, 2) click on the "effect" tab, 3) select noise reduction and follow the instructions, 4) click on "get noise profile", 5) select the entire clip, 6) click on the noise reduction tool and select okay on the step 2 section, and 7) listen to the file to see if you like it or need to adjust the effect's settings, when you are satisfied with the results export the audio and save it as a WAV file.