Loud knocking/rattle at acceleration, Chevy dealer says engine ready to go?

What is your question?

At 124,219 miles, and literally 1 week after Chevy dealer replacing the GM factory recall of ignition switch, the engine light illuminated once, oil level down, and loud engine knocking/rattle at acceleration started. Chevy dealer says the engine is ready to go (suspect a failed bearing or rod)? No diagnostics, just 'listened' to the engine. What test(s) can be done to narrow this down vice an engine replacement (quoted $5K for a new engine)?