Sometimes there is constant background noise that isn't an electric hum but constant crowd patter. I used to go to Soundtrack Pro to do this. But I can't with FCP X. Is there a way to do this I don't realize?
As always, big virtual high five to you guys. This site is a lifesaver.
Try the background noise removal filter, which comes up after you ask X to analyze your audio. If that doesn't help, export the audio track and use the free program called Audacity. play around with that, then re-import/replace the audio. or use Apple Soundtrack pro if you still have that.