The sound effect very end?
No clue! A video I saw said it was like Tony Stark building the Iron Man, but I don’t know what that would imply.
My half assed crazy theory is it’s Gamora (the one Thanos sacrificed) trapped in the soul stone, in non-corporeal form, and the plot of GOTG Vol. 3 is Quill’s quest to find it, and get her reunited with her 2014 body, so she will remember him again.