Just Need A Little Help Please...

I recently had the whole head of my car replaced with newer high performance parts and now theres a knocking at 2,500-3,500 RPM. Its only when you press so far down on the gas pedal to...If you let off the gas the knocking goes right away. I've been to a few mechanics. Most have told me its slapping valves/lifters... The guy that did the head work said its something in the bottom end like a wristpin (but a wristpin would constantly knock right?). And the most unlikely answer was that its something in the intake. Theres no lack of performance from the car but Its just a little annoying that I have such a beutiful car with a knock in it. If anyone can help with this please let me know...