I’m not an acoustics expert but it could TWO sources of sound converging at your house. Have you asked any neighbors if they hear the same thing or would that cause them to think you are crazy?
Trust me - as likewise not an ‘expert’, but an amateur audio engineer who took to designing/building subwoofers of a type simply not on the consumer market - it’s assuredly a summing of waves. The irony is that I believe it was somewhat self-inflicted: I ventilated my house last year with gable vent/fan and that’s about when the most regular ‘sound’ began. Worse, a detectable ‘hum’ in the 10-15hz range manifests when my gable fan is on, purely from airflow through my rafters. Maddening, in a word.
It worsens when the local train trestle gets freight traffic and when certain types of vehicles pass through the area, but there’s one singular source of infrasound you can ‘sense’ at certain spots in a walkabout of the surrounding area and which is nearly constant in parts of my house.
I’m exploring whether I can build some Helmholtz chambers out of Sonotubes small enough to fit in my crawlspace to effectively trap the waves before they reverberate in the living space. But absent finding the source - and doing anything about it - it’s just another motivation toward the property I desire a bit farther out of the area and more isolate (where I’m at was intended to be transitory, <5 years, but 2 more years of this is nearly unbearable without mitigation, which might include disabling of my new gable fan just to keep my sanity).