Loud rattle when idling in the water

[soccerdad]

We put the 96 GTX (Shamu) on the water for the first ride of the season. It keeps fouling the Mag side plug. I think that is just extra oil that seeped in during the winter. I did just rebuild the carbs but I don't think I messed that up. But it also has developed a very loud rattle, almost banging sound, when it is idling or at low revs when sitting in the water. It seems to go away when I have it under power. When testing it only would get up to 5000 rpm, which I expect is the plug fouling. I didn't want to blow it up, so we just pulled it off the lake. It has a solid 150 PSI front and rear thankfully. Any ideas on the rattle? I rode it without the seat and see nothing moving or loose. Could it be the baffles in the water box have come loose? Or could it be something in the rotary valve area? I am going to put it on the hose tonight and look more, but any ideas would be very helpful. I will shoot a short video of it tonight if I can't determine anything.

 

[POPPS]

Your trailor rpm should be 2500 rpm,,,in the water 1500...that shatter can occur from a low rough idle,,.a leaky oil seal could cause a lot of smoke,,,but no rattles or banging,,,could be a crank /rod bearing,,,almost sounds like detonation,,,to rule out the broken baffle, just remove the waterbox,,,and shake it.