I am thinking #2 was sleeved but that was 25 years ago when I rebuilt the motor so my memory is a little fuzzy,it looks like I will be dropping the pan and pulling the head to get #2 piston and rod out to see what is going on and report back. If alot was not done to that block I would just run it until it blows but I want to see what went wrong.
Don't believe it will KABOOM, based on your descriptions.
You have spoken of a "rattle", not a "knock".
I'm still thinking of a timing issue or a runaway, loose piece of carbon, bouncing on top of a piston.
Time will tell.