08 c6 coupe. 140k miles. Engine was making a high pitched rattling noise with hard acceleration. Speeds up with rpms. Closest thing i can compare it to would be similar to a loose heat shield. Couldn't find anything on the exterior engine causing the noise. Thoughts went to a bad lifter. Pulled the heads. Lifter trays are in tact, lifters appear good, visual inspection of cam shows nothing worn worse than any other. Haven't found much slack in the rocker journals, lifter rollers, etc. I don't see anything bad enough to cause the noise. Any ideas???