I have a certain shotgun mic that pretty consistently picks up an 8k whine from one of my ikan floor lights. I don’t hear it in the headphones but it shows up in post. Easily eliminated with a notch filter in audition so I don’t sweat it.
I’ve checked the clips in FCPX as well and it shows up there too so, for me anyway, it’s not the editor introducing the noise.
Check your clips in another editor or audio editor if you can and try a notch filter on it since it’s a pretty well defined frequency.