Chevy Knock.

2000, 1/2 tons chevy w/ 118,700 miles, 5.3 L. It started when I added sea foam engine tune to the master cylinder vaccum line. The engine wasnt running when I added this. I tried to start it after adding the sea foam and the starter would try to engage, but would not turn the engine over. Then put two starters on it and tried to shim each one. Still wouldn't turn over. Next we removed the plugs and turned the crank pulley with a ratchet. While the plugs were still out, we turned the engine over with the starter and it turned fine with no knock. We then installed the plugs and tried to start it. When it took off, it was knocking very loudly. I assume it is probably a rod, but I have used sea foam before and have spoke with many people who have used it and had good luck with it. Has anyone heard of sea foam causing major internal engine problems? I also was told that it could be the flywheel or maybe something else. I hauled the truck home and the mechanic was scratching his head when he heard it run. It has only been ran a max of like 20 secs or so it this condition to try and keep the damage at a minimum. Let me know what you all think and lets hope it isnt a rod, other wise I am in the market for a n engine. Thanks for reading!