Back when I was trying out lucid dreaming, getting the ear-ringing like you described was the first step for me to transition into a dream while awake. I've noticed other redditors mention the ringing in /r/luciddreaming sometimes but it's never really talked about. It's spooky when you first try it but honestly it's really relaxing and calming when you get used to it. I've never had any problems sleeping after trying lucid dreaming because just getting that ringing to a loud volume as a whitenoise just lulls me to sleep (and if it happens, the lucid dream is nice too).

Reading above, I find it really interesting that what we're hearing is apparently electrical impulses. I remember reading a bunch on beta/theta waves and how they come into play when dreaming: I wonder if by controlling the muscles and "raising the volume" of the ringing, we're actually doing something to the level of electricity running through our brains? I always get into the hypnagogic state of lucid dreaming extremely fast when I put the ringing on full blast compared to when I just lay there not thinking about anything so mabey there's a correlation, idk. Super interesting stuff.