Sorry to be “Simon Cowell” here but IMO there is no way of fixing that intermittent distortion.
Not noise reduction, notch filter, equalization nor Holy water will cure it, sorry.
It’s a milder & less frequent version of the distortion corresponding with the bass which occurs with bootleg rock concert recordings , e.g. “drove ole” …
Whether the music is Classical or Rock there is no cure for this distortion: the distorted imprint the bass has left on the singer is indelible. Filtering out the bass, or lowering the overall level, now is like shutting the stable door after the horse has bolted, (if the bass or overall level could have been attenuated at the time of the recording that could have prevented the distortion from happening, but it's too late now).
If you re-record with the same set up again you need to reduce the sensitivity of the video camera’s microphone, and perhaps move the microphone further from the piano and closer to the singer.
MLK3329 wrote:It was an oval rotunda with a high domed ceiling and marble floors and walls.
there may be
resonance effects at the performance venue which amplify certain frequencies.
When doing a test recording at the venue use a piece of music which includes a wide range of frequencies, (e.g. scales), and when recording leave some
headroom (a safety margin) just in case the performance becomes unexpectedly louder due to resonance.