Any guesses as to the source of this metallic knock?

Engine is a Chris Craft 283 - which is basically a Chevy 283 Small Block but set up with the flywheel forward and the drive shaft at the timing chain end.

Here's the YouTube video. You can hear the knock clearly. https://www.youtube.com/watch?v=OnUSIOs7iH4&featur...

The knock is intermittent, both in that it doesn't occur every revolution when the engine is running, nor does it occur every time I run the engine, so I'm pretty sure it's not a spun bearing. The sound 'seems' to be coming from the forward left side of the engine.

I have checked the flywheel - solid - and all the flywheel bolts - tight.

I have checked that the starter is solidly bolted on and not moving.

I removed the valve covers and ran the engine. All valves are operating as they should, all with the right amount of travel.

I've looked inside each cylinder with an inspection cam, no loose objects, none of the spark plugs were damaged.

I've removed the intake manifold and looked inside each intake port. No loose object, no apparent damage to the valves or sign that anything has been rattling around.

I removed the inspection plate on the side of the oil pan. No loose objects or metal chips/shavings in the bottom of the oil pan. Both a magnet and a clean rag came out with no 'gritty' stuff.

I removed the oil filter and cut it open. No metal bits in their either.

Anyone got any other ideas? Pulling the engine is a last option because it's a hard top boat and I can't get a crane over the engine hatch.

Richard