Continuous hum is one of the kinds of noise that can be addressed.
This is the kind of “continuous” noise that a program like Bias “SoundSoap” can help remove.
https://www.kenstone.net/fcp_homepage/review_soundsoap.html
Bias has recently released “SoundSoap 2”
https://www.bias-inc.com/products/soundsoap/
SoundSoap was retailing for about $50 (up to $80 just before the new “2” release).
SoundSoap 2 is priced at about $100.
You export the audio, process it through SoundSoap 2 and re-import it to FCP.
OTOH, If you have FCP 5 Studio, Soundtrack Pro has “sampling” noise-reducer that does the same operation as SoundSoap.
Lastly, if you don’t have either program, you can try using the standard FCP “Hum Remover” filter, but it usually ends up eliminating most of the WANTED audio as well.