2014 Sierra 1500 5.3l whistle solved

[Cg86]

Posting to hopefully help someone else. About 10 months ago I noticed a whistle coming from the engine bay. It would only present while in gear and driving, I could not get it to happen while in park, so I had a very hard time determining the exact location of the culprit. I started replacing belts, pullies, CAI, vacuum lines ect... to no avail. 

 

Soon after the truck started to idle rough and would occasionally misfire. Changed all the plugs, wires, coils, injectors, intake gaskets with no success. I took the truck to the shop and was told it was an AFM lifter, but at that point I had already had the valve covers off and was fairly confident that was not the case. 

 

Well about a week ago I took my go pro and zip tied it in different locations in the engine bay to see where the sound got louder, and it seemed that it was coming from the passenger side, aft part of the engine.

 

Well lo and behold after pulling the heat shield off there was clear evidence of an exhaust leak from the aft manifold (I do not recall it being there when I was changing the plugs). A quick visual inspection showed no broke bolts or obvious evidence, but when I went to check the torque of the aft most bolt, it immediately snaped off with no force. I used a drill and extractor and was easily able to remove the sheared threads left in the head. 

 

So after months of stress, hours of labor, and thousands in unnecessary parts, it was an $8 exhaust manifold bolt.

 

Afterwards, no more whistle, engine idles smooth, and I seem to be getting better mpg. 

 

Hope this can help someone else.