It’s called infrasonic noise or sound. It is the effect from ground waves or vibrations, not air waves, and it is low frequency which may account for the hum or whatever. You feel it as much as you hear it.
Yeah, I’ll bet a lot of these reports involve trains, hills and bedrock.
The surrounding hills can deflect and absorb the train sounds, the granite could connect the low-freqs to the ground, where it could travel and radiate.