Unfortunately, i do not spend much time in meditation.
My progress therefore tends to zero, at least i consider it to be so.

But when "doing" meditation, i am getting into an unknown state , which is reproducable every time.

Maybe you have an idea what it could be and how to go on "further".
I prefer to lie down rather than sitting with closed eyes. No mantras. Just trying to think nothing.
After about ten minutes, when i lose (only) partially awareness about my environment,
something like a silent but loud gong (i can not help to describe it in other terms) comes into my awareness.
Still my eyes closed, i get then the feeling that i am now in an other state,
since it is definitely different compared to the one when starting with meditation.
I would say i am more awake and alert and more focused in physical reality then before.

Either:
- i was about the lose physical awareness and due to unknown reasons i snap back and therefore feel now more alert

or

- i am in a state where i should go on and see what happens
(I have tried this, trying then to focus on me, on others, on past future etc., but it seems that the only result if i continue further is falling asleep)

I think the best way to go further is to just do your meditation, whatever happens.
It's possible this is subjective, these 'experiences' while meditating.. For example - I don't hear a gong, I sense some 'rush of energy' or something going through my body.. which lasts for few seconds (when I'm not repeating a mantra).

This is maybe the same as your gong, maybe not.. but the best way to find out what it is, is to stick with meditation, to see what other 'changes' would occure, either related to this gong or some other ones.

If this didn't happen to you before, this gong sound, then apparently there's some movement.. and I'm thinking, forward.

This isn't very helpful, I know .. maybe other board members can give you greater insight about this.
Anyway.. keep at it.

I think the "snapping back" is common. Since your not used to being in that state it is a natural reaction. I remember the first time I heard myself snoring I did'nt realize it was me at first, now I can meditate through it and when I shift completely away I don't hear it. I have had snap back problems from the beginning, I just worked through it and that increased my meditation time to a couple of hours a day.Everyone seems to experience meditation differently, troubleshooting from the outside is difficult.

I have a 14 yr old Dachsund (Corky)who meditates with me.When I get to the vibrational state he starts getting agitated and restless every time so I just have to ignore him and go on.I can almost tell when he's going to start getting fidgety and sometimes I'll just bust out laughing and have to start all over again.A lot of obstacles in meditation.Corky started barking during an OOBE the other day,there I was flying through the neighborhood with Corky on my back barking away, it was quite freaky.

Sorry I don't have any real answers for you, just anecdotes.Meditation seems to be such a uniquely personal thing it's hard to know what will work. I hope you keep working through it and become successful.

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Shut up and meditate