It may be infrasound. Had some experience with infrasound a decade ago or so. Extremely annoying for people than can 'feel' it. We detected the source, a large ventilation fan, using geophones. I remember watching a video of a vertical bar that oscillated because its natural frequency matched the frequency of the infrasound; of course, I can not find that video now...
With a pan and some salt, maybe some nice Chladni figures can be obtained: