What would make it? It almost sounds like a rod knock but it is before it starts and while cranking. Engine runs fine and it has only done it a few times. Starter maybe? If I can get a video with the sound I will post up.

My guess would be slight hydro lock of the engine which the starter is able to overcome. Your exhaust may be leaking water into the engine while sitting.

It could be rod knock, happening before the engine builds oil pressure. Had an old jeep that would do that, until it finally spun some bearings.

Check the easy stuff … I had a couple of loose bolts on the idler pulley that sounded exactly like a rod nock.

Agreed check the easy stuff 800 ponies could rattle stuff loose

Had a WP pulley knock while cranking. Loose screws. When running sounded fine.

Yes look at all rotating parts on the front. Then check starter and could have a cracked flex plate good luck!!!!