2007 trailblazer ss 2wd. 64,000 miles. remote turbo, with zr1 cam, texas speed push rods 150ibs manly springs. at approximately 2,300 rpms it has rattle sound similar to pinging. it doesn't happen at idle its not pinging. its not a constant noise. driven about 1,000 miles since the noise started and its still the same as day one. the car could sit for a week and once you start it no noise at all. only comes in once it heats up. we took off the valve covers and ran the car and the noise didn't change. it seems to go away in lock-up but that just may be since the rpms drop a little we dropped the oil and it was free of any shavings or anything. Motor is immaculate once the covers came off. we have checked all of the following :

1. Accessories - took of serpentine belt ran car
2. torque contver - no loose bolts
3. fly wheel - not cracked
4. unplugged each cylinder 1 by 1 noise remained
5. checked for fractured rockers. all look amazingly new
6. checked for bent pushrods. all are dead straight
7. put screw driver up to timing case. noise still the same.
8. Didn't check lifters. dont feel like pulling the heads to find nothing wrong and still be stuck at square one

im a toyota mech (no i don't like em) and my friend who's a bus mech are beyond dumbfounded. we are at such a loss. we need help. Please anything to help us with this would be great. thanks

 

Im thinking that its a broken valve spring, at least double check that.

Some of the symptoms you say negate this, however i still feel that it could be the issue, as it is sometimes hard to tell if they are broken, examine them very closely.