I have a 05 monte carlo LT that I'm doing a rebuild on the engine. I'm not sure if it's a piston loose or something else. Hopefully somebody else has had this happen and can tell me.

 

That sounds very much like mine. I haven't tore it down yet mine sound like the problem is in the valve train. Good luck on yours. If I get mine tore down and find the issue I will share it. I just can't decide weather to tackle it myself or get another powerplant.

 

That's a pretty loud knock!

Like GTHSS suspects, it could be a valve train issue, i.e...lifter, but IMO it sounds too loud for that. I'm thinking a spun bearing...main, crank or rod.

Were knock sensors activated to throw a code?
Did it quiet down at all when warmed up?
Did you pull plug wires one at a time with engine running to check for knock from any particular cylinder?

 

Do you have an update on your engine trouble?

I think I have my problem diagnosed and temporarily fixed. I took the advice of a mechanic friend of mine. He said if it is just a stuck valve that I could buy some time if this trick works. I raised the hood the other day when it was warm outside and let the sun shine on the engine all day. Just before the temperature started to drop outside, I cranked the engine and immediately the engine was not making near as much noise. As I let it run at idle within 2 minutes the rattle went to a ticking two nothing. After that I added some Marvel Mystery Oil and it let the car idle for about 30 minutes and drove it around the yard some. It is running perfectly quiet and not missing. So evidently it was a stuck valve and the warmer temperatures allowed it to free up and get some oil circulating. I know this isn't a fix and other problems will probably arise but at least it can be a grocery getter as long as I keep oil and water in it and baby it. Maybe it'll buy me enough time to make a better decision.

If your problem is a spun bearing or other bottom end like plumbob suspects this trick will not help. Good luck and let us know how it is going!