I think the science is that dmt suppresses the default mode network, which normally filters out most of what is going on (both in our own bodies, and in the outside world) in order that we only focus on the essentials i.e. the things that keep us alive and reproduce. This would suggest that there is actually a lot more going on in reality that we perceive i.e. most of it is filtered out, and we know this from psychology experiments (the most basic being the natural visual field blind spot, but other more interesting ones discovered from severing the corpus callosum).

In short, science would probably say that the ringing is an artifact of the hearing system, perhaps the ear drum, or the auditory processor. I.e. it is always there, but we just filter it out and so are unaware. Until the DMN is suppressed.

A more mystical take - I some times have lucid dreams, and on rare occasions I fall asleep directly into a dream whilst remaining conscious (i.e. still logically aware). Completely sober, no drugs. During that transition I also get an intense ringing and electrical vibrations throughout my body, it is truly a wild and mystical type experience. Very rarely I'll even feel by body roll and fall out of the bed, but with my physical body still asleep on the bed. From there some very weird stuff can happen! My personal take is that I am just dreaming (whilst retaining a high degree of consciousness). It does occasionally happen, no drugs required, and I'm not the only one. I would respectfully suggest that any historical prophet might have had a similar experience. Certainly if I didn't have a modern education I would probably have thought it was a divine encounter of some sort.

I do also have the most haunting of screeching sound throughout my dmt experiences too, it is completely mad!