Mine made a sound like that when one of the exhaust valves let go and trashed 3 more cyls.

If its a rod knocking it won't have any oil pressure. What's your oil pressure at idle? It don't sound like a rod knock but its hard to tell threw speakers.

Sounds like a rod bearing to me. I spun one in my old truck with a 305 (originally I thought maybe it was the transmission because it was waaaay low on fluid, turned out to just be a coincidence), here is a couple of my videos for comparison:
http://www.youtube.com/watch?v=yx57kMkpIXM
http://www.youtube.com/watch?v=mQZFcyccfb4

The squeal is the rod bearing working its way out between the rods I think. Made for some good pictures when I tore it down.

Sometimes I would lose oil pressure until I rev'd the engine, then the pressure would come back, other times it was just fine. When I pulled the engine and tore it down the crank was wore down about .040; I knew the engine was bad and needed replaced but I figured what the hell, one more run to the landfill won't hurt it any worse....:D

I also did the pulling of the spark plug test, no change was heard. The knock was the piston slapping the head in my case.

Oil pressure is normal. No issues there.

I guess u will have to pay $75 bucks to get it looked at by a mechanic at the closest marina. It could be lots of things. If u can take some stuff apart and put them back then go with some suggestions givin here. Pull valve cover look around. Use a long screw driver with your ear on the end and listen around with it. If u got good oil pressure it can't be a rod knocking since its that loud!!

I have not read the entire thread.... If it has not been said, the first thing you need to do is too is take off the oil filter and cut it open. If you have any metal, more than likely you have a bearing going bad.
And yes you can have good oil pressue and have a bad rod bearing/s. Seen it several times, you might notice a small drop in oil pressure.

Yep had a 350 with a rod out the side of the pan and 30 psi oil press at idle. Throw that theory out

Ok there big boy what ever u say!!

Ok im very confused over here.

We pulled the engine and started it out of the boat and the sound disappeared. We also removed the outdrive too. When we put it back in the boat the sound came back. Removed flappers and its still knocking. Did the spark plug pulling and still loud as ever. Removed the belt and started her up and still knocking. Open the valve train all ok. What is going on here? I messed around with the screw driver and my ear and it sounds like it is coming from the intake. No leak from exhaust manifolds either.

Mechanical fuel pumps can make a noise like that. Sometimes sound can travel through other components and confuse you as to its source. I would cut the oil filter open. Fairly simple process - any engine shop should have the proper tool to cut the case open.