Looking at the spectragram, it looks like 3-phase 60Hz pulse modulations (the black dots in the spectragram) spread across some harmonics in the audio band.
I'm still guessing some big honkin' industrial welder or similar.
I can get a deeper look once IT gets my good audio analysis SW with 3D spectragrams installed on my new laptop. This is from freeware Audacity:
N164WM RV-6A Slider/O-360/FP Sold 4/2022
N184WM (RV-8) Ready for inspection. Titan IOX-370/ CS/G3X
N184P reserved (RV-8QB) Empennage kit and QB wings in the shop