You guys were helpful before, so I'll ask my next question about this engine...

Here is the video:

http://www.youtube.com/watch?v=BbNICZHNo9k

Here is when it happens:
- Engine warm
- NOT at idle
- most noticable at 1800rpm up to 2200, maybe is also higher but can't hear it
- under load or not, doesn't matter

Here is what I have done to diagnose:
- preloaded Lifter arms 2 times engine running hot (1/2 turn past quiet).
- used stethescope on engine when making noise (nothing jumps out)
- eliminated fuel pump, rocker arm ration, bad fuel, timing, too lean, too hot, carbon buildup, and flexplate bolts.
- oil is clean, and oil press is 25 at idle, moving up as rpms go up.
Engine construction:
- all aluminum heads (newer match set from Holley systemax)
- hi lift, long duration cam (Holley systemax)
- old headers with paper gaskets (tightened when hot)
- rockers looks good and the pushrod guideplates are adjusted.

Do you hear it? Any ideas what it might be? If it's just normal noise, I'm ok with that, but I do not want to be doing damage to the engine. Below is a video clip of the engine running with valve covers off so you can see rockers.

http://www.youtube.com/watch?v=FuoUpnFBKK0