Many hums can be connections to realms which are wholly psychedelic/etheric in nature. I am a psychonaut who has seen many things and different “hums” out there in the spiritual. They are very present in what I would call “Realms of consciousness”. You could be hearing a hum from a source which is not beside you in a scientific sense. Jeff Daughtery talks about astral holographic projections and drawing people into an illusion with that skill. The astral is easily broken apart and accessed if you smoke DMT(ayahuasca works even better but is more difficult to prevent possession/interaction within) or drink a large amount of mushroom tea or peyote. The thing that smokable DMT has on a few things is that it injects you directly into some part of the astral or even subtler realms that you have no idea about all of a sudden. That can be good if you want it. Destructive if you aren’t ready. I personally can do all of the proper faculty without help from DMT laden plants BUT that stuff can provide a tool/bad shortcut. Once you are in many astral realms reached VIA DMT or self projection intent you will hear a background hum.