Steps:

Drag and drop your video clip into Audacity
Go to Effects and Normalize.
Go to Effects and Noise Removal as seen in screenshot above
Select area of noise and click Get Noise under Noise Removal. Note that any operations get performed on a selection. The noise area is the area between known sounds, for example when a person is not speaking.
Then select whole clip. Use Ctrl-A. To select the whole clip and Remove Noise - just by hitting OK under Noise Removal.

Audacity Notes

Noise Removal from Sound