Does a knock plus dropping oil pressure mean a spun rod bearing?

It's the classic sign of a failed rod bearing. A bearing that has worn or spun opens up the clearance so much that the oil pump can't hold normal pressure, and the loose rod knocks.

What usually causes it: low oil pressure over time, or a short period of severe oil starvation, damaging the bearing through metal-to-metal contact. In a spun bearing, it overheats, grabs the crank journal and turns in the rod, which destroys the bearing's locating tabs.

How to confirm: the knock is deep, comes from low in the engine, and gets louder under load. Disabling one cylinder at a time (unplug its coil) often quiets it on the bad cylinder.

What it means: major engine work. Thicker oil or a quick bearing swap may quiet it for a while, but by the time it knocks the crank journal is often scored or out of round, so new bearings alone may not last. Plan on a crank inspection or regrind or replacement, and find out why it lost oil first.