300k miles on it could be the answer as well as the suspects that have been listed. Attach an oil pressure gauge and tape to w/s and drive it and watch.
For what you need an oil pressure gauge?
The rattel noise of the engine is evidence enough.
I would not drive one mile with this car, because a low oil pressure up to engine rattle, is a Guarantee for a massive engine failure soon.
I would remove the oil pan, check for dirt, swap the oil pressure relief valve and the o-ring and look what happenes.
If this not fixed the problem, I would change the oil pump.
Or what I would do, just because I'm so keen to disassemble and rebuild an LS engine, after all that fckn Audi/VW or BMW shit, which I rebuilt recently...
I would pull the engine and rebuild it with love.
But not everybody can do this himself, has the tools, knowledge, experience and capabilities or money to give the engine to trusted Garage.
But as far as I know rebuild engines are in the USA not very expensive, compared to an engine for an German car. For example a Mercedes AMG engine cost up to 60,000 euros and last mostly only 150,000 kilometers...
What a waste of money....
So every saved LS engine is good for environment and for the legacy of a time where people could build cars....