Thanks for the replies. Out shooting for a couple of days. Now I’m working my way through all the filters to see if anything helps. This is some nasty noise.
I had already tried the basic filters under Audio Enhancements. Background Noise Removal did nothing for this loud rumble, and Hum Removal didn’t help, AC hum wasn’t the problem. It’s the Other AC noise. I did find that checking the Loudness control for all the clips boosted volume in a helpful way at no cost otherwise.
I tried Denoiser but couldn’t seem to isolate the particular sound well enough. What finally seems to help after much trial and error is the Remove Low Frequencies filter, tweaked as best I can manage, and then the audio enhancements Background Noise Removal on top of that. At least it sounds somewhat less as if there is a jet preparing for takeoff somewhere inside the interview location.
The Background Noise Removal worked very well on a different clip with a bit of much lighter hvac noise in a different location.
Would be nice if FCPX had a noiseprint capable filter.
Thanks again for the replies.
Best,
John