I have a 1994 Jimmy 4.3l. (Am I allowed to talk Jimmy on this Forum?)
The engine has about 160,000 miles on it. It was running very well, until it developed an exhaust manifold leak on the left side. We commenced to replace the gaskets. The rear manifold bolt was broken off. Due to its inaccessible location, we removed the head to get at it. It was very easy to remove The bolt after removing the head. The bolt was replaced and everything was reassembled new gaskets and all.
When we started it , the engine had a loud knock, that was not there before disassembly. It sounded like it was in the upper, front of the engine. It ran, just making the noise.
We removed the intake manifold and found a roller lifter that had disassembled. The lifter was replaced and no other problems were noted, so it was reassembled. The loud hammering was not there when it was started, but after a short time, the noise came back. Now I am starting to run out of fresh cuss words. Does anyone have suggestion for what is causing that. I am about to give up, because I am not going to pull the engine and rebuild it since it is so old.

 

You can talk Jimmy on this forum, same thing as a blazer or bravada, just a different name.

As for the noise, are you able to get video of it and upload it?