As one who can hear these sounds, I can attest to how maddening it is. It's even worse when (most of) the people around you can't hear it, and they don't believe you. Earplugs only make it worse, because they filter out ambient noise that otherwise helps distract you from the incessant hum. Daytime is not as bad as night because of the natural activity sounds of a city. I have read that waves and underwater currents…
Short version: I made many attempts to track it down. Used low frequency Microphones, filters, amplifiers, etc, as well as PC Spectrogram programs to try and and visualise and record it. Visited many nearby industries etc to try and find it. But none of my attempts did any good.
As I'm a Radio Engineer I spent ages experimenting with VLF Receivers, Spectrum Analysers, etc.
One thing I noticed however was that the Hum was much worse when I was living in a quiet rural area.
One night however, I noticed that I could affect the hum by doing the swallow maneuver that divers use to equalise ear pressure. I slowly came to understand that my Hum was an internally generated sound.
Apparently there are various possible mechanisms: The simplest is blood flow near the ear. But also the tiny hairs in the ear canal actually have tiny muscles which cause feed-back or "regeneration" to make our ears more sensitive, and the frequency response of each hair cell much sharper. However at times, this regeneration can slip over into actual Oscillation.
This ability of the ear to actually generate sounds is well documented. It can actually be loud enough for a Doctor or a partner to hear. It's called Objective tinnitus.
In my case, the Hum eventually disappeared and has never returned. I suspect that it coincided with a change in blood pressure medication, but I can't be sure.
Looking back, I would suggest that it may be worth trying to amplify the sounds via a set of high-quality earphones and appropriate filters, etc, using the headphones as microphones.