The hum is fascinating. In looking this up I see that people still hear it even with ear plugs. Massey University in New Zealand managed to record the hum (link) .

On this page is a clickable to hear the sound. When I listened to it, my whole backbone vibrated to it. It felt so amazing and resonated with me (even made me feel emotional ). Thus I theorize this is related to spirit/earth.

The hum we (me and D) have been hearing here lately is in the morning when it is still and there is no traffic outside. It sounds like (the apparently proverbial) diesel truck running. It is not anywhere near as loud as the recording. Maybe the amplification of the hum is why the recording affected me so much and the morning hum here does not.