Engine knock, and oil presure drop after warm up
 

new to the forum, have a question, think i already know the answer, just hope someone might have another suggestion, or thought.

my 05 silverado, just started knocking, and the oil presure drops, the strange thing to me is that, I shut it down, and when I start it back up it sounds ok, for a few mins, then the oil presure drops, and it starts knocking, or starts knocking an the oil presure drops, not sure which.
I assume the worst, and its a rebuild, but wanted to get some thoughts before i tear into it?

i'm assuming the worst, and its a spun bearing, piston slap, rod knock, something major, but my quesiton is, why would it sound ok on startup?
shouldn't it sound the same immediately if there where a spun or thrown bearing, or rod?
I'd think the knock should be there soon as I start it up, not wait until the oil presure drops, or is the oil presure drop casued by the knock?

hope someone can understand this, any thoughts?

please let me know,
thanks,