Mmm, I noticed it when I was really young, so I'd say since I was maybe 8 or 9, I'm 25 now. It would go for days, weeks and months at a time. As I got older I would notice spikes after having long music making sessions or being at clubs, drinking a lot, and heavy exercise (which was rare when I was younger, lol) but it would subside pretty quickly - to me it was very, very normal because it had literally been there most of my life and I didn't track it too thoroughly. I think body position and stress would play with how loud it was too, but I can't remember too well!
It's always strange for me to think about it now because it definitely was a form of tinnitus, but because I had it since I was so young it had absolutely no negative impact on me.
Only thing I could relate the humming I experienced to is that I grew up with enlarged tonsils and adenoids (lots of ear infections), and also found out recently I have a deviated septum, so maybe something was pressing on my eustachian tubes or something to that effect.
But yeah, I'm not sure why it went away but it did go away around the same time I got what I now consider tinnitus, multi-tonal high pitch frequencies mixed with all sorts of noise! So maybe the two are correlated but who knows really.
If it continues to worry you, get your hearing evaluated by an audiologist (if you haven't already) and then maybe visit an ENT to possible rule out any physical factors that might be influencing your hum, you could probably do this with an MRI.
If you have any more questions or anything, feel free to pm me!