Help... 6.2 Denali engine noise
 

Ok so my truck has been running fine, 2009 Sierra Denali 6.2, 185k miles, oil changed every 5k. Tune and muffler only. Was driving to a friend's house, it was fine, no noise at all. Pulled up, shut the engine off, then when I went to crank it, it was making this noise. Engine shakes a good bit and running really bad. Sounds like it's in the valve train, I'm hoping it is. What should I do when I get inside the engine? I tow a little bit, have 33" mud tires and a leveling kit. 3.42 gears awd. Really don't want to get a converter due to not having much to spend. Really just need to get it fixed but figured I might as well upgrade since I'm in the engine. I drive a good bit, got the truck about a year and a half ago and have put 55k miles on it. So needs to be reliable.

Perhaps a lifter is stuck or a valve spring broke, it's top end for sure though.

I'd pull the valve covers and start checking things out to see what really happend.

Just got done doing that, no Springs are broken. I'm thinking towards the lifter. I have a p0306 cylinder 6 misfire code

From what i can tell. It sounds like a rod knock.

Sounds half the speed of the crank to me, you could just pull the dipstick and if it's covered in glitter, then the bottom end is the issue.

Lifter let go pull the heads is the next step. Inspect the cam really close to see if it is galled or pitted if it is anything but perfectly smooth gotta replace the cam. I don't remember which ones did or didn't have DOD they are all running together too damm many iterations these days. But if it has the valley plate that is not smooth on top it's got DOD. I'd delete the DOD. Install standard lifters and tray's and smooth valley cover turn off dod in tune and rock and roll. As long as the cam didn't get ate that's going to be your cheapest route. Dont get lazy or try to save a buck and do one side only you will only cost yourself down the road. It may be worth pulling the oil filter and cutting it open before you do all of that work to see if it's even worthwhile to pull the heads. If it's full of metallic paint shes screwed pull the motor.

Head is pulled off, valve seat dropped and it hit the piston but not very bad. Got lucky. Very strange though. Never heard of a valve seat droppinh. Will post a pic when I get time

If it shattered the valve seat you need to replace the intake manifold and the Y-pipe assembly unless you want further trouble down the road. It sent pieces through both ways that can migrate back into the engine under varying conditions and cause a recurring failure.

Odd failure but at 200K miles almost no one can say it didn't go the distance.