2005 Hummer H2

Hello Hummergirl
noise can be complicated to diagnose, but here is what we can do.
Have the drive line checked.
I have few questions
does noise the noise occurs when accelerating or coasting? If yes the problem is typically in the driveline or chassis.
I know the gas price is high right now, Are using lower octane in your hammer than the one you should?

What I can see for a high pitched rattling noise that intensify during accelaration is the ring noise.
It can be caused by a worn ring or cylinder. Other causes included broken piston ring land and too little tension of the ring against the cylinder wall.
Ring noise correction requires ring, piston or sleeve replacement. In most instances it will require boring the cylinder.
When a vehicle overheat, anything can happen within the engine events for few minutes.I'm not sure if that has caused the problem.
If the oil pressure is good, then the problem may be the rings.

Does it have any accompanying roughness, and can you determine which side it is coming from? Could it possibly be an exhaust leak at the manifold to head? Often times bolts break due to expansion and contraction over time, typically the left side rearmost(#7 cylinder). Have not seen any rocker, or lifter issues on H2, or other 6.0l powered trucks, other than those someone has created by improper repairs. The heads are aluminum, and it would not be unrealistic if a rocker bolt/stud has(is) pulling from threads. If you can determine which side it is from, the cover is not too difficult to remove, and may lead to results.