Join Date: Mar 2013
Location: Grayslake, IL
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It's been said a couple times already but it sounds very much like a valve trane to me. You said you had the rocker arms off? It could be a possibility that the valve lash was adjusted very loose however I think at this point things have progressed beyond that... Pulling the valve cover and doing a thorough visual inspection would be where I would start.
Inspect to make sure there is not any rockers that are obviously loose or damaged. I would also look at the jam nuts and make sure none have backed off. If that happened the adjuster will back its self off and the rocker will hammer the little foot on the end of it to death.
If you do not see anything obvious try barring the engine over and both feel for any abnormal resistance or abnormal noises like popping.
While barring war h your valve trane to make sure all valves open and close smoothly and none of them hang up or stick.
Also one thing to try is remove the rockers and using a brass hammer tap the valve stems... You don't need to hit them hard just tap them. A normal one will feel somewhat springy when you tap it and have a ping sound.... If you get one thy is bent it will be pretty obvious.
That will hopefully give a place to start... I had a C15 generator set that at a out 235hours had the exhaust valves on cylinder 4 meet and bend The valves. The sound with the engine running was very similar to yours. The cause was the jam nut had not been torqued properly from the factory and it backed off, the adjuster backed off, broke the foot on the rocker and bent the exhaust valves.
2006 Mega|Edge CTS|Fass150/Beans Sump|EFI/CSP5 Anarchy|2nd Gen Swap 63/68/.91BW By Anarchy|5" Exh|305/65R17 MickyT ATZ P3